Because as we speak the internal GPS on my android is seeing only 3 satellites, very weak strength, and 3072m error...
My external GPS (much more sensitive) is seeing 11 satellites at different signal strengths and is locing on within 3m...
So based on these numbers the external GPS is 1000x better! ;-) Really though you can generally get much more sensitive external GPS units than the ones built into the phones. They can also be located somewhere convenient where they will get a better view of the sky. Plus they have a seperate battery and therefore take less juice from the phone.
From what I've been able to gather Android's bluetooth stack does not support SPP (Serial Port Protocol) and therefore can't connect to a BT GPS on its own. I did find this project that provides this support, but you'll need a custom app to take advantage of it as it's not tied into the Android location system.
http://code.google.com/p/android-bluetooth/
Source post here:
[android-discuss] Re: Can Android use an external Bluetooth GPS unit?
Hope that helps!