Traffic awareness, I'd guess. I use Google just for that whenever I'm away from home (which is a small city, I don't have to go far, and I know all the alternate routes if one is busy). I already know how to get to 90% of the places I'm going in some cities, but they're the easy-to-remember routes, not necessarily the fastest ones.

Presumably they have a dense network of roads in-between, where the optimal route can change daily.