Because it's been so hot out during the day lately, I've been going out in the evenings more when it cools off a little bit. Movies with a view are much better in the dark, and participating in Central Park's Film Festival is easier when the sun starts to go down (especially when it's raining).
Even shopping on a Saturday in the city is easier when you start around 5 or 6 pm. Stores stay open just late enough, but not late enough that you shop too much or as much as you would when it's early afternoon (this is good)!
One of the greatest things about nighttime in the city is the view of illuminated building tops. The lighted points and edges make great photo opportunities and are of course interesting and beautiful.
The Empire State Building, seen from 21st & 6th
Met Life Tower, view from Broadway and 23rd Street