From what I've heard they're not really necessary until 3rd or 4th year when you start doing clinical rotations. PDAs can run procrams like ePocrates (which tells you drug interactions, dosings, contraindications, side effects, stuff like that), MedMath (every medical formula you could need), Surgical Recall and other Recall series and I think even the PDR (Physician's Desk Reference) is available for PDA. Supposedly there's also patient tracking software, but I don't really know anything about that.
Anyhoo, you could see how it all could come in very useful in the hospital, esp. in tiny little PDA form rather than several dog-eared and pocket-sized books! 🙂