When Amazon launches an API everyone cries. Same story over and over. Even better example: TakeAway-Group. The perfect MITM.
The owners of German supermarket and car companies are really the richest of the rich in Germany (okay and maybe the SAP guy on top). It would definitely be a net positive if someone manages to scrape and compare their prices.
In the restaurant market it's one player abusing many small players.
And honestly, I think the reason everyone cries when "Amazon launches an API" is because Amazon would not dare to piss off the German supermarket oligopoly.
There's a few projects doing that for DE / AT at least.
The issue is that each market sets their own prices and I believe REWE is the only large one where you can fairly easily scrape the product catalogue. I thought about it in a shopping list context, so you'd need to make it location dependent to be useful. But you could do a lot of cool things with it. Like choose a basket of goods and it creates a route for you: "Go to supermarket A and buy goods XYZ, then to supermarket B and buy ABC"