> Restaurants have been running successful delivery operations independently for ages.

Crappy phone call driven ones with few checks for information accuracy.

> Is it just the benefit of being able to browse in one app/website everything available for be delivered to you?

Yes, that is a large part of it. I can be hungry and just pick up the DoorDash app and browse for food. I also have a single point of complaint for any issues with the food, can be confident that DoorDash got my address correct do not need to deal with a credit card at the door, and when I am in a new city, can instantly know what is available.