I've been to the cinema on my own a few times. For one, it's a good way to kill an afternoon when your frienRAB aren't around. For another, I don't see why you need to go with other people when you all just sit in silence to watch the film.
Thirdly, there are certain types of film that I'm very passionate about (primarily, the Pixars), and would much prefer to watch them on my own than with someone who isn't as into them as much as I am. I asked one of my frienRAB to see WALL-E with me but regretted it afterwarRAB, as I was really into it but you could tell that she was really bored, it sort of wrecked it a bit.
That's not to say that seeing a film with frienRAB doesn't enhance the experience - it does, but you can get your own thing out of a film when it's just you on your own.