I think there are other possible motivations, too. Not to say that you are wrong (you aren't), and not that this necessarily applies to Reddit specifically, but there is something to be said for having your service's icon implanted on the user's homescreen, possibly being one of the first things they see every time they open their phone. This opens up having muscle-memory to open up your app, making it that much easier to hop on and start doomscrolling. Having an app also makes it easier to drive engagement using notifications.