I collect because I really enjoy movies and posters are an extension of flickers.
And I'm also drawn to the amazing artwork that so many posters provide, and that has made me seek out those movies that I have not yet seen, whether it's a domestic or foreign film. The movie could certainly be a stinker, even if the poster is uber fantastic.
That's what it's all about for me. Exploring and discovering movies I haven't seen before (or revisiting ones I havent seen in forever). And tripping across cool and beautiful posters inspires that, so it's like a win-win!