The RAW file format holds much more information than you can see with your eyes. Sure, shooting in in JPEG leaves you with smaller size photos (in terms of storage), which results in more photos on your memory cards. This results in faster transfers to your PC, and Photoshop is more responsive to smaller files. So there are numerous occasions where JPEG seems more than enough, until you do need that (little) extra RAW information....
Just watch the following video.
Nuff said, I think.