You do realize a hobby is something you get pure enjoyment out of, right? Otherwise it would be a job or a career. Most people who bodybuild will never make a career out of it, but will do it because its fun.
I actually disagree with you when you say its completely useless. You build your body, you diet down, you learn a hell of a lot about your metabolism and body in general. That's a wealth of information 95% of the population will never find out about. And if you like bodybuilding enough, you can find a way to make money out of it.
When I was a teenager I used to love programming. It was a hobby. I wrote code, when it worked it was such a rush, and when it didn't the challenge of debugging it was really satisfying. I decided to make my hobby my career, and here I am, a programmer making money doing something I would do for fun anyway. Then I found lifting weights and bodybuilding. I got in touch with a supplement company after my first show and now work for them part time, making money using another hobby.
Its all about how you use your skill. Either do it for fun and nothing else, or find a way to make money out of it. Both things are perfectly satisfying and useful in life. You need to do something that brings you joy, and something that brings you money. Those don't always overlap, but when they do you've hit the jackpot in life.
