It depends on your line of work. My first 6 years in the game industry were grueling 80-100 hour weeks for years on end. It was actually worse in the ~4-6 weeks of light work (40 hour weeks) between projects because you just can't adjust to that quickly.
If you didn't have a good friendship with the people you worked with, life would be fucking hell. I ate lunch with and played cards with co-workers every day. I ate dinner with co-workers every day. I walked to 7-11 to get cheetos at 2am because the fucking PS2 CD burning keeps failing and we have to try again.
Kind of unique industry, which I am still in, but I make it a point to be friends with co-workers after going through that because you don't want to be in the office at 3am with someone you fucking hate.
I don't think I've ever hung out with co-workers after work. I try my best to keep those relationships as platonic as possible.