A place to pour your bucket of excrement.
I loved sewers. Perfect place for a child of the night to hide out and have fun in the day
When they had toilets, that'd be true, but when towns/cities were at the bucket stage, they typically just tossed it out the window on whomever was walking below. The sewers could've been to carry water from storm drains, I suppose, but I don't think they had storm drainst without having paved roads and by that time, cities had toilets.
As to a place to hide out, it'd be nice if you could get an actual place down there as a home if you were a vampire or whatever.