How come you'd need to buy a seperate GPS thing to use GPS...couldn't the phone just use the signal to find the nearest tower and use that as a general guideline as to where you are? I know that wouldn't be as accurate...but it would help sometimes.

is there a reason they don't/can't do that?

8100 does not have that capability (8800 does for example) so you will need to acquire a BT puck in order to use GPS. Here is a link by Stinsonddog. He is the GPS goto guy:

Stinsonddog's Blackberry GPS Tips