Some people are in it for the challenge. Someone else's outage? Yeah I'll happily help diagnose it, analysing and figuring out where the issue is can teach you a lot of things.
For the record, this was never at night. Late in the evening, sure.
I do that as well, since I am my own business
Also sometimes (depending on the company) being seen to visibly chip in fighting fires can win you real credibility that you can put in the bank and then use later when you want to slightly slack off for a bit.
So it can actually make logical sense to do it occasionally even from a purely selfish perspective if it's half an hour on a random Tuesday evening and you aren't actually doing anything else important.
All depends if the company is actually going to be grateful or not though