I don't understand the distinctions perfectly, but as far as I can tell, Favor is a service of people who will walk into a restaurant and order whatever you want.

Grubhub is, I think, more directly connected with the shop. I think they put the onus on the shop to do delivery. Favor is their own delivery service.

It all blends though- I think Caviar is basically Favor, and they're pretty close to TaskRabbit For Food Pickup...