I'm curious if they are actually a major discovery destination.

I suspect most customers are either starting looking for a restaurant they know by name, or one near them to ensure maximum freshness, using Google Maps or Yelp or the like. Only when they've already chosen a restaurant do we transition into the delivery app flow.

I'm sure a lot of money is being spent for chain restaurants to advertise "order via Postmates" or "exclusively on GrubHub"-- again bypassing them as a discovery system.