The answer would seem to be pretty obvious; for health reasons, to improve appearance, to be fit.  But if you dig a little deeper the question becomes more interesting.

Since I was a kid, for 50 years or so, I’ve been  going to gyms.  I generally don’t know the people there or have any interaction with them, so all I can do is observe. 

A gym is a unique environment in that the same people tend to work out at the same time of day as a matter of habit.  I see lots of fit people in gyms where I work out.  I don’t measure their reps or weights, but visually, they don’t seem to get any fitter over time.   In some cases, by the time I see them, they’re already very fit, and just maintaining that level of fitness may be enough.

But to me, working very hard and being in the gym a few times a week is a big price to pay if you’re not actively improving in some way, whether that’s strength, appearance, health – anything.

It also seems terribly boring to me to workout without goals.   Going to the gym and having a clear idea that you’re going to try to get 7 reps instead of the 6 you got in the last session makes things a lot more meaningful, and over time, those incremental results really add up.  I’m terrible at chin-ups, but each workout I try to get one more than I’ve done before, and, eventually I do.  If you don’t reach your daily goal, no big deal; having a goal will make you push harder, and have better workouts even on days you don't hit your targets.  If you set realistic goals, over time you’ll achieve them.