Well, sometimes, you're a big fan of a movie and really want to see it. I was really into World War Z so I went and saw it when it came out in theaters, the first night. Other times your friends all want to go see Star Wars together, or some other friends and I went to see John Wick 2 the night it came out.
Sure, for the most part I don't care about going to the theater, most movies suck anyway. But socializing is a thing and watching a movie on the couch just doesn't compare to being in a dark theater with the screen filling your entire field of view and feeling the rumble from the speakers.