For me, maps and directions are the best thing. Sure, I can figure it out myself, but this is just far easier.
For example, recently I had to go from Hongo to Jimbocho. I've been to Jimocho a few times before, I roughly know how to get there, so it would've been a simple matter to walk to the nearest station, look at the subway map, and figure it out that way. Instead, I search from "my current location" to where I needed to go on Google Maps, and it recommends me to take a bus instead which was much faster.
I'm out and about, and suddenly get a hunkering for sushi? Just search for the nearest kaitenzushi. Need directions in an unknown city? The taxi driver in that foreign country doesn't understand which hotel you're telling him to go to? Show him the map on your phone (previously I would always print maps for this purpose when I travelled, now I get to save paper). And there's no need to carry around fold-out maps or Lonely Planet guidebooks everywhere I go anymore either.
Couple that with the ability to read using the Kindle app when I don't have my actual Kindle handy (e.g. waiting for the supermarket check out), or quickly check Facebook (or indeed Channel 9) whenever I have to wait for a bit, and I think it's pretty useful. It also doubles as an MP3 player when I'm working out.