1. Recruitment - You may hire someone you meet.
2. Networking - Someone may work in a related industry that can be advantageous to you.
3. Relaxation
4. Keeping aware of industry trends. Yes some of this can be done online but in-person gives a different perspective and can be...
5. Fun.
We went to some HN London meetups in London when we were at an earlier stage (hard to find time now). It was fun, a good way to unwind and meet other like minded people. You don't have to throw 100% of your life and time into working and sleeping only.
Why would anyone go outside and meet people, have a drink or two and talk about common interests? That's also beyond me.
As always, there's a happy balance. It is probably bad to be a social recluse, unless you are in crunch mode and not looking for hiring anytime soon, but at the same time it's probably bad to be out for events every night and spending too much time socializing.
Some people go to meetups to socialise with people working in a similar area. It's probably good to de-stress with something like that even if you have an early-stage startup.