Theaters need the studios to make content for them to show, and studios need theaters to show their films, especially for that huge opening weekend box office that generates all the buzz that gets other people to go see the movie. It’s a huge thing for theaters that they have an exclusive on the movie for several months. This means if you really want to see that hot new movie that came out a week ago, you have to go to the theater. And studios go through a series of stages of availability - first it’s exclusive to the theaters, then it’s only available to buy, then it’s only available to rent, then it’s only available on some premium service (like HBO), then it’s on a streaming service like Netflix, and finally it may make its way onto broadcast TV. At each step, they can make money off the people who are comfortable with that stage, but, not willing to pay for the previous stages. And of course, sometimes they get repeat sales, like the person who sees it in the theater and also buys the BluRay.