It's obvious there is value, otherwise these companies wouldn't exist, no matter how much VC money is pumped in.
For consumers:
Ability to order from multiple restaurants through one consistent interface / payment flow. This cannot be undercut by every restaurant with a phone and a teenager with a car.
For restaurants:
A marketing / lead generation avenue that provides, ideally, incremental volume that is profitable. If it was not profitable, then they wouldn't do it, obviously.
Delivery itself, is just a method to deliver these value adds.
The argument can be made whether this value is worth a tech infrastructure and the human labor cost of delivery. It might be worth it in China, where delivery is actually more ubiquitous, but in America, where worker compensation / expectation / norms are higher, its debatable.