See http://electronics.stackexchange.com/questions/11884/how-many-gps-channels-make-sense/

That long answer details how adding more channels to a reciever (at additional cost) can:


  -  Speeds up satellite acquisition
  -  Reduces power consumption
  -  Reduces likelihood of losing a 3D fix even in urban canyons
  -  Provide better sensitivity, allowing fixes in dense forests, and even in some tunnels
  -  Provides better positioning accuracy