Most docks are little more than a plug with some extra plastic, but the Nexus One dock, recently leaked by the FCC, has a bit more.
Bluetooth, in particular, is a notable addition in the dock (since Bluetooth is already in the phone). In other words, you're docking a Bluetooth device into another Bluetooth device. Why could this be?
Really, there are many explanations—and Bluetooth is cheap enough at this point that it need not be a luxury. Maybe HTC wanted you to have a headset that you kept at home. Maybe Bluetooth was chosen for remote accessories. Maybe Bluetooth just sounds cool to engineers.