When you say 'an external bluetooth device', are you talking about a bluetooth USB dongle? If so, the correct software should have come with it. From there on you can (as you point out) sync with your mobile, send MP3s to the mobile (if it has the capability), connect to other bluetooth enabled PCs/laptops for data exchange, print via a bluetooth enabled printer...etc.
Bluetooth can be considered as the removal of wired connections between devices.
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