> I can't understand who would want to drive in Manhattan.

I think the idea is that people will drive if their tolerance level for slowness is satisfied. So the amount of traffic will always be high enough to look acceptable for some people, unacceptable for others.

As for parking, in London there are definitely parking lots scattered around the city - can't speak for New York though.

> Like where are they coming from and where are they going?

Probably going between their homes and family / friends / shops / entertainment venues / restaurants.