I've bought a humber of movies because they're ones my g/f and I really like and will/have watched multiple times (TDK, Princess Bride, Monty Python Life of Brian and Holy Grail).
Others we bought because it was the same price to rent them. The 4 for 20 deal on used movies at Blockbuster. It's cheaper than renting if they're in the 5/20 section. We've gotten good movies that way to.
Also, series. like BSG or BoB where I don't want to have to run out to get the next disc.
And for clarity- I don't use Netflix, and probably never will. Just not my style. I don't really watch enough movies, and when I want to rent something, I want it now, not tomorrow or 3 days from now.