Well, 2 reasons...

2d art - mainly this for now. My wife and I are needing to start doing some drawing and art that will require a tablet.

3d art - and this is more of a fun hobby for me :) Plus I'll use it for my game design later.

I'm not really a good hand drawer... so I'm worried that the disconnection between where I'm drawing and where I'm looking will be a problem... that's why I'm heavily considering getting the Cintiq 12WX - because I can then look at what I'm drawing.