You train because you get stronger in the process, and getting stronger makes everything else easier.
If you’re in high school, this means you get better at your sport. The stronger version of you can now generate more force than you could previously, and you’re now a stronger, more powerful athlete who can practice harder and longer.
But as we get older - as we enter our 50s, 60s, 70s, and beyond - this newfound strength has a far more profound effect. It is absolutely life changing. Getting stronger changes the quality of your life, and the logic is pretty straightforward.
You’ll note that your training never gets any easier. You absolutely get stronger, yes, but you simply add more weight to the bar, so your training is always challenging. But because of this - because your training never gets any easier, what you do out there - in life - does become easier.