My reasons are:
I want to sleep on my own schedule. It really improves my immune system.
I want to be physical everyday. This is harder in the winter when I'm at work during all the daylight hours.
I want to plan meals and cook.
I want to learn what I want, when I want: French on Duolingo, trapeze class, creative writing course.
I want to read... a LOT of fiction books, and do a lot of research on the web on various interests and current events.
I want to write a book about what I do, but it's hard to find time to write about it while I'm still DOING it. Also, I don't want my job to consume my every waking thought.
I don't want to have to choose between doing one of these above OR being social, because all I have is a few precious evening hours.
In short, I want to do whatever I want, whenever I want.