Health is your choice.

What can happen if you're not doing any sports?

If you're not doing any sports in old age you're going to at least be weak. Because your muscles are not going to be strong, and your bones will crunch a lot.  Also if you are a kid and you are crunching a lot with knees/arms or whatever part of your body it will hurt. That's why doctors advise you to do sports in childhood.  But for sure doing sports is not only healthy and all of that stuff it's also fun, it's interesting to play with your friends. But exercise also affects mental health (read about it on the next page)