The Disciplines are systems that I have studied and practiced over the years where they have morphed into a cohesive blend for me. Each related to the other in some aspect and helping the others. Different and yet the same. In each there is but still that feeling of needing to work on more to improve things in each. A student for life (a double entendre). Continued practice leads to improvement in each of the other disciplines, which adds to the whole being. Practice of them is to aid the process of “Not Getting Old as you Get Older”
You need to have different disciplines to combat the bodies natural ability to adapt to specifically imposed demands. This is called the SAID principle which stands for Specific Adaptation to Imposed Demands. As the body is placed under some form of stress it will make adaptations that allow the body to get better at withstanding that specific stress. If we keep the body under different stresses at different times it gets better in lot of things as it adapts. There is an overall improvement, not a specific improvement of one thing. If you just practice golf your game will improve, but not much else. Adding other disciplines will improve your golf game more or what ever your main emphasis is.
If the body feels good, the mind feels good. If all feels good, then you feel good. When you feel good, your spirit is strong. A strong spirit is very good.