For the same reason ppl with CDs listen to their music as mp3s on iTunes - common sense really.
No offense intended, but please explain. If you have a DVD player, or in your example, a CD player right there, why wouldn't you just play the media you already have instead of spending your time ripping it to another format? I don't see the common sense in that.
It does make complete sense for other things. You obviously can't use your music CDs in an iPod, so you have to rip them to mp3s in order to take recordings of your music with you. Ripping a DVD can also make sense if you are a frequent traveler and want to bring movies to watch on a laptop with you, without having to drag your original disks with (something movie studios have
finally figured out why people do that and include digital copies with your purchased movies for just that purpose).
But ripping a DVD to watch on your TV, when you could much more easily just play the disk seems like an utter waste of time.