Basically, the USP for these platforms (like many of the kind) is being always up-to-date on the latest from the stores you like. Fear on missing out great deals is what keeps the users engaged, frankly.
With the push notifications from the app, the users are provided with weekly coupons, deals and whatnot. Eventually, you start awaiting that content.
What do people think of them? Depends on who do mean - the end users don't perceive them as marketing platforms, obviously. For them, such platforms are aggreagators.