Originally Posted by
canklecat
Also, most phones can use cell tower data as a backup when GPS sync fails. I don't know whether there's any bike computer that can do that. This may explain the occasional significant variations I see between my phone and bike computer logs.
GPS is much, much more accurate than the location from cell towers.
GPS takes a while to get accurate location. Starting with a good approximate location allows GPS to start up much quicker.
Often, the location is needed on phones much quicker than GPS can get it. That's another reason cell towers are used. Phones can also use databases of WiFi networks to get location. Both these (cell towers and WiFi) also work better indoors.