That way with bluetooth on for example when you get into your car and it will connect to the car's bluetooth system automatically. When you exit the car and get back in after getting gas it will re-connect and allow you to play music without having to struggle getting it re-connected. Otherwise it take me about 5 attempts to reconnect, and it eventually will, but is not very efficient.
Ideally the part of having to know the devices mac_address should be managed in the gnome bluetooth settings itself (since it already knows the connected mac_address), which would simplify the second part of getting autoconnect to work.