Good point. But Yes and no ;)

Grub-less et.al are as much about ordering and checkout as they are about discovery. I can rank and view all restaurants offering Sushi in my delivery radius. This is what makes them powerful.

Uber et.al are a marketplace. You are absolutely right in that they have zero switching costs but they have a two sided pool of supply (drivers) and riders (demand). This is hard to replicate.

In Europe takeaway.com took over lots of delivery apps and businesses. In Germany they took over foodora and deliveroo exited the market. takeaway's platform is from a feature and UX point of view not justifying any premiums. You can't tip drivers, which is the most important feature enabling contactless deliveries. There are no new features since years. You can literally filter by cuisine and price; nothing else, not even a map with restaurants nearby, search by dish, etc. People order there because it's the only choice, but everyone agrees the page is just awful.

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.

How exactly is that different?

There is an argument to be made, that if I just want a pizza, then my major decision-maker will be price.