Yeah, theaters don't make much from the ticket sales, since film makers like Disney takes 65% of the ticket sales. But, you will likely spend more on food and drinks or other stuff for sure, and that's where theaters makes most of their money. When you bring people to the theaters, a lot more goes on for the night out.
Some people might want a lift from Ubers for example, if they ended up drinking a bit on the night out.
McDonalds might have a special offer for MP members, things like that to generate more business. When I went to see Thor two days ago, I went to local Apple Bee restaurant and it was packed, I bought drinks but no pop corns for the movie show in the theater. I needed food and drinks lol. Easily spent 50$. If I didn't go to movies, I would not have been to the local restaurant, and I would not have bought drinks.
So yeah, this potentially far bigger than just being able to go to movies for a low monthly subscription. Because you don't have to worry about other movies that might be bad one if you had to pay 14$ for a ticket. You end up more willing to try out other movies that you would normally not want to pay for them. It most certainly helps those mid tier films and not just the top tier big players.