There are many good reasons to build a backyard pool. Just looking at your pool or listening to the water has a calming influence. Maybe you'd like to maintain fitness and good health, as swimming is great exercise for all ages.
If you enjoy entertaining, then your pool is going to give you a reason to do more of it. The pool can also help establish a theme in the landscape and set a mood.
The real question is, why wouldn't you want a swimming pool?