Not every game needs to be that, sure, but maybe Remedy want to make, and enjoy making those types of big AAA blockbuster narrative based titles? If they do, development costs for those types of games by Remedy's own admission, are ever increasing, so higher sales in some ways is indeed necessary. Not to mention they're a business so they'll want to make as much money doing what they love as possible.
I'm not saying I agree with this acquisition (I don't know Remedy's financial situation), but I can see why it might make sense for them. Quantum Break didn't exactly set the world on fire with sales, so perhaps they believe that with Sony's experience, consumer demographic and marketing, they can make the kind of game they want (eg have the budget for it) whilst simultaneously having greater financial success?
It'll be interesting to see how things pan out. I'm personally not expecting Control to sell particularly well. The viewing and interest metrics for it have been pretty low.