@Neverpraying: I buy so many because I then loan them out to friends and family. Think about this. How much is the theater these days. Around $10. That's just for one ticket. The average DVD costs $15, and you can watch it as many times as needed. Hence the loaning out. Also it's the physical media aspect of owning a disc as opposed to Netflix or Itunes.
I'll just be yet another guy who has seen all the other posts saying the same thing as mine but still want to raise my post count without admitting it. Either that or I'm an idiot who wants to make it really clear to you what the issue is, as if all the other posts which say the exact same thing had not yet made it clear to you.
Collecting. Also you can get back to them and re-watch them whenever you like, it's pretty cool.