Very useful for getting value out of games you no longer want and also useful for building up a catalogue of games. Like I've got an enormous stack of Xbox 360 games that I got for cheap points and haven't even got around to playing yet, including Dead Space 2, Dragon Age 2, etc. Lots of cheapo "budget" titles too, but hey, I'll play them eventually. It's my version of renting, basically.
I also use Goozex as a way of giving gifts to my gaming buddies. Like I spent a few points getting all of us copies of L4D and L4D2.