Here is my main reason. Apple's DVD Player is bad at deinterlacing to a degree that is as baffling as it is inexcusable. Some interlaced DVDs actually look better with deinterlacing turned off. For these, I use VLC. Otherwise, though, the features of Apple's Player are pretty convenient and VLC can be awkward and flaky.
As to the why: I like having other alternatives. One of my friends had VLC fail on him, and he kept having some weird problems with DVD player, so I thought it would be nice to see how other players would react to the dvd.