I always thought watching films that steam in the dark streets was some cool 80s aesthetics. As in 'You are in this dark corner of the city where nobody can hear you' scary or edgy.
In addition to the heating/cooling uses, the mint in Denver uses it to clean coins!
There's a stretch of sidewalk in the Financial District that gets so hot that you can feel it walking past on some days. And apparently, sometimes these things explode... [2]
As far as I've heard it's not (all) actual leaks, though, but rather water dripping on the exterior of the hot pipes carrying the steam itself being evaporated.
[1] https://en.wikipedia.org/wiki/New_York_City_steam_system
[2] https://en.wikipedia.org/wiki/2007_New_York_City_steam_explo...
Maybe the steam pipes keeps the sewers warmer in places with steam but they should be well insulated in the end.
i don't think it's steam leaking. it's drainage/sewer water being heated by the steam system (and other heat sources down there) enough that it evaporates and rises up to the street, and then condenses into vapor/fog because the street level is cooler
Apparently it's to regulate pressure.