There are many reasons why indiviudals buy the same film multiple times. Some may do it for the extra features (meaning extra fun for them). Another reason could be that they are wanting to sell the dvd's to other individuals or maybe they want to give them to their friends or families. It could be because they want to burn the copies and they have 5 computers to do so (you never know they could be running an illegal business). As I have stated before there are many reasons why individuals do so many different things that other people just think about. You must remember, everyone is different.
I've only given in to it once - I bought Blade Runner: The Final Cut after buying The Director's Cut the year before( replacing the old VHS), which I got in impatience waiting for this long-awaited anniversary edition to come out. But, it was worth it for the improved sound mix & improved tech used to redo many FX shots, plus the extras.
"I'm not smart so much as I am not dumb." - Harlan Ellison
I do it for better extras, transfers, etc. I also do it if I really like the packaging of a certain edition. I know itís not really practical having multiple copies but once you start buying collectors editions there are lots of cool editions out there and itís easy to get tempted!