My thinking is that they'd buy it to harvest the data for AI training.
The user experience is pretty much secondary to the strategic play here.
Reddit's value is in the body of data its amassed, not in the ongoing service to users it provides. Limiting 3rd party access is all about protecting that and has the added benefit (to them, not the users) of creating a walled garden where they can increase the (meagre) margins from advertisors by controlling the data flow to users.
Not saying I am a fan of all this, I'm just recognizing the situation.