I suppose the obvious answer is: because you can afford to. If you have a bit extra left after paying the bills, then you should spend some of it on what gives you pleasure. With me it's listening to music, looking at movies & reading, so it's CDs, Blu-rays & books (& a bit of alcohol). I love owning these things & try not to take them for granted...& guys love collecting, women not so much, I suppose they're more sensible. I think in general the younger generation aren't so hung up about owning things.
I own upwards of 600 movies. Why? Because I just love movies. These days I don't get to watch them as often as I'd like but I still love knowing I could when time is available. Today time isn't available because there's football and also my wife's priorities which included breakfast, a trip to the mall and "talking" later. She knows it's football season so maybe this was passive aggressive behavior?
I now have "The Sacred 300" as an estimate, followed by the "Nice to Have Around" 100, followed by recent movies or new acquisitions I may or may not keep in the long run. I'm always weeding out new and old that I doubt I'll ever view again.
I also collect books, so between these two collections I have two great "comfort foods" for any mood. Some movies I watch several times a year, others maybe every five years. I'm glad they're within reach.