Imagine carrying your small and lightweight iPad on your hike. You start questioning the route you are on, so you pull out the iPad, where you can reference Tom's electronic version of his book. You read where Tom says to avoid the class 5 rock by traversing climber's right of 2nd tower on the east ridge. You aren't sure if this is the 2nd or 3rd tower on the ridge, so you pull up photos that you saved to your iPad prior to the trip showing the east ridge, dang... still can't tell for sure. Now you launch your GPS application that overlays your position on a topo map, viola, you see that you are on route.
All would be available without a cell signal. I know that makes you purists cringe, but that would be some cool stuff right there!