I kinda wondered this too, but my best knowledgeable guesstimate is that the cell towers are there to assist in GPS navigation, hence "aGPS." It's much quicker for the phone to get a signal from towers, than from satellites in space, so by getting a good idea of it's position based off the towers, the satellites pinpoint the phone once a proper signal is acquired. The other thing I read was that the GPS location of the cell towers is recorded for guidance as well, so the phone's not going "OK where in the world am I?" but more like "Alright where exactly am I between Tower X,Y, and Z?" However unless you have some way to save the maps to your phone, you're going to need the cell towers to download the map data anyway.