Why? Because the iPhones GPS chip is HORRIBLE. Face it, none of the apps can keep your location locked well enough to have reliable turn by turn. The TomTom basically turns your iPhone into a TomTom, by adding a more powerful GPS chip. Save your money if you are going to be getting GPS software. I have tried them all, and they are all horrible. Its a shame they made the GPS antenna or chip so anemic. Driving home today G-Map, and the new Sygic lost track of where I was several times and took a while to recalculate. xGPS doesnt have even a sniff of these two either. That likes to put me in the middle of neighborhoods and lakes! My friends phone does the same thing, as well as my 3G... I currently run a 3Gs.