A primary motivation for physical training is development. Development is the notion that one is getting better at something, and therefore, is becoming more powerful than before. The more you work on developing yourself, either physically or otherwise, the more powerful you can become. The more power you have, the more potential you can gain and then apply.
In the realm of physicality, being strong physically means that you can do more things independently of external, hired help. This, in turn, can save you money. Thus, physical power is one that can save you other forms of power, such as finances. In exchange, you keep that finance and get to spend it elsewhere. If it weren't for your physical prowess, you would depend on one or more hired hands that you could use without.
Hence, why the development of physical power is the development of your inner resources. The human body is a center of much promising potential. Physical exercise is an exercise in unleashing that potential within you. Neglecting your body means neglecting that potential, making it underdeveloped, and making you dependent on external sources of power unnecessarily.
The body is a tool that you own. In a sense, it is your most basic asset. Developing it further extends its promising potential beyond its current practical territory.
Physical exercise is a skill like revamping a philosophy article. The more you do it, the better you get at it. The better you get at it, the less painful it can be for you. Therefore, it is a reasonable interest to develop your body to reduce or even eliminate pain that may be included in physical tasks. Not developing your body means exposing it to weakness, and the weaker you are, the more likely you are to suffer critically and unnecessarily. Strength, physical or otherwise, is also a source of defense from such agony.
To protect yourself from such agony, you must endure the agony that is required for development. Be afraid of that vital agony, and you will fail to invest in protection from harmful agony. Vital agony is one that contributes to your development, while harmful agony is one that hinders it. There is no life that is devoid of discomfort, and thus, from the potential of agony.
Live a life that is devoid of vital agony, and you will expose yourself unnecessarily to future, harmful agony. This includes any other area that is not physical. That's because vital agony nurtures strength, and strength builds character. In order to have good character, both in strength and virtue, you must entertain vital agony. Physical exercise is the exercise of tolerating vital agony on the physical level, and that tolerance can be applied to other departments of our lives.
The carefree hedonist who avoids agony may find themselves suffering from harmful agony as they failed to build strength against it. Strength is not only used for offense but for defense as well. As such, the emotionally strong person would be one that can use their emotions practically, like the physically-capable man or woman. Being emotionally strong does not mean, necessarily, that you are an impractical drama-inducer. No. Strength is also expressed in the wise allocation and application of resources. This is why wisdom is an expression of strength.
For example, the stronger military force is not the one that is larger in quantity. After all, such forces have been defeated throughout history by smaller armies. Thus, strength is not measured in intensity or quantity alone. It is measured by its ability to exert its power in order to either control or influence.
What is the practical point of a man or woman on steroids who does not have the power to clean their homes because they are on overdose? What is the point of a large military force that is also hungry and tired in the middle of battle, against a smaller force that has been well fed and rested?
Fun fact: Asceticism is derived from the Greek word "askeō," which means "to exercise" or "to train". The ascetic trains the power that exists beyond their own willpower. As such, a good ascetic is one that can operate independently of willpower. They develop a specific power that can be exerted despite many situations. This is what makes them very disciplined.
Combine reason with asceticism, and you can operate in this world as long as your actions are reasonable, with little else required for that intention. With little inside of you (desires, temptations) standing in your path.
Training is more than just developing our bodies—it's the maintenance done in the name of health and iron will. Training is needed even regardless of one's unwillingness. Do you take the garbage out of the house? This is a prime meaning of training—to clean ourselves also from negative traits such as clumsiness, childishness, and slothfulness which threaten our physical well-being. Do so regularly, and the "house" that is your body will be clean from these traits, like a literal house.
In the absence of training, the body will unnecessarily deteriorate and can become a liability in our paths in life. Maintaining it through training would mean that it will get less in our way, and be more than an asset. Like a pet, it requires regular attention and caretaking. But unlike a pet, we're necessarily dependant on it as well.
When we neglect our body, our positive values may become hurt as well; values that can be used for departments outside of pure physicality. We can become fatter and more tired, and at times even less disciplined as well. It can affects interpersonal departments, like when socializing. Like when looking for a partner, and so on. It has little to do with our own body image, and more of using our bodies to advance in life regardless of our perception of it.
Rubinshteinic philosophy holds practicality as one of its supreme values. The ultimate point of practicality is to survive and thus to thrive. Thus, treating our bodies like practical tools can further our survival in life, and allow us to better thrive. All our actions begin with our bodies and we cannot do anything without using the body first. Even our minds are part of it, so we can't even think without using our bodies.
Developing the body means developing the very center, the very foundation of all our actions. A good foundation should be able to support more successful and promising actions. This is why a healthy mind can definitely exist, at least more than otherwise, as a result of a healthy body. And of course, we need our minds intact in order to allow good cognitive actions.
How can a philosopher be a good thinker if their minds are distracted by constant ills and harmful agony? These can damage the clarity and rationality of their thinking. This is because there are constant interactions between body and mind, whether or not the mind is purely physical.
The metaphysical essence of the mind does not matter as much, because practically, it only matters that the mind is a product of the body. Even if it exists independently of the body, it cannot operate independently of a body, rendering it ineffective when bodiless.
(By "mind" I may also refer to "soul").
As we train, our muscles grow and become stronger by resisting gravity and the objects we hold. This is because our body is here to be adapted to the environment it interacts with. The more interaction, meaning, the more it resists its physical surroundings, the stronger it will become in the name of surviving in the environment it is put against.
Human beings have become the conquerors of Earth mainly because of their ability to adapt to their environment. This is why developing the body is excellent for the purpose of adapting. The muscles build themselves accordingly. They are our extensions and they are there to assist our living.
If you wish to become stronger by training, you have, first of all, to resist your urges that are used as obstacles and excuses. Second, you need to accept the importance of vital agony in the name of its untapped benefits to your interests. I believe many people give up on training because of the inconvenience of pain and sweat. However, pain and sweat are a direct indication of development!
Development is based on overcoming former states, AKA, growing out of them. A tower is built by stories overcoming their lower levels. The self is therefore built on overcoming its former versions. It cannot develop without successfully overcoming what it used to be. For muscles to develop, they must overcome their former, weaker states. When you race and beat your previous time scores, you know that you have overcome your former records, and thus, the former versions of your body.
With the hedonistic values that our Western society possesses, we become weaker and more fragile, almost to the point of self-injury by unhealthy substances. All in the name of feeling good. Ironically, such good feelings can lead to harmful agony in the long run, in the form of poorer health, and its consenquences.
Fun does not have to be the primary source of motivation. A good source of motivation should be justified by its practicality. Good feelings are not as practical as survival and maintaining our health. What would happen if training stopped being enjoyable? Would you neglect your body because your counterproductive feelings told you so? Unfortunately, many people would do so. Treat your training as if it were a habit and a duty, and you can survive better by being in better health. In return, you can also operate better in life.
All in all, training is a means to an end. It is a "necessary evil" used to maintain and/or improve our physical power. And physical power is a "necessary evil" in itself, because in this world, the strong rule over the weak, and the mighty dictate what is deemed right, what is deemed normal and desired. As such, being physically big and strong can help you overcome certain types of adversity, even if by display or behavior alone.
Extra note: Since I'm a big and strong man (see the picture), people very rarely interfere with me in real life. They know, even by intuition, that it is unwise to meddle with me. The sad truth is that people of smaller sizes may have it harder in life. So, at least by being more muscular, you can make certain people keep their distance from you, like a scarecrow may do to pests.