I'm hoping you know what the a in aGPS stands for. OK, good. Now look it up on wikipedia. Ok, now that's out of the way (you read the entire page, right?). Obviously the assistance can come from multiple sources, and the device (ie phone) can do anything from having a full GPS chip set (such as most modern phones, ie, the EVO) and using assistance to simply speed up it's GPS lock, or it can know practically nothing about GPS and have the assisting service do all of the heavy lifting. The Airave has to assume that devices connecting to it are as dumb as they get, and it has to be able to provide GPS coordinates for them whenever they can't provide their own. This would be one of the reasons why it needs it's own GPS lock, to provide that service.
Sprint could have opted to require you to input your physical address on a form, on-line, every time you power cycle the Airave before it allows any calls through it, so that you can verify your physical location (think Vonage adapters), but that's annoying and not very user friendly, especially since the Airave doesn't otherwise require you to even own a computer to use. Thankfully, they didn't do that. It's a poor hack anyway. If Vonage wasn't so cheap, they'd put GPS in their adapters too and this requirement would go away for them, too.