I think if you want to understand and appreciate movies, then you can't just watch the films you want to see. You don't have to see every movie like a critic, but I believe it's important to have a wide variety of experiences.
Often this might just mean seeing something when it's on TV or on Netflix, where you don't have to pay to see it. Why someone would want to actually pay for a product that they believe they won't like, though? Well, maybe it's for the reason above, and they like to see things in cinemas rather than waiting for home release. Maybe they are eager to see just how bad something is. Or maybe they simply like being in a cinema.
I saw Taken 3 partly because I wanted to see how bad it was, but also because it was a nice couple of hours to get away from my family. My cinema has super comfy seats, and I always sit in an area where virtually no one else is sitting, so it's just nice to relax. Granted, I don't have to pay for films since I have an unlimited cinema pass, but I can easily see why someone would spend time watching bad films, so I imagine some people are prepared to pay a little money for that too.