Plug in a cord for a phone charger.. and use the 5v off it to charge the phone.
THAT, is about it 😞
Generally, the port is there for 'future usage' type thing which often never comes along.
There was some documentation on the PREVIOUS version where there was suposed to be in the long run DLNA playback/server, which might have used it.. never came around.
I GUESS that you should be able to use the USB for a HDD extender... though its HIGHLY recomended to use a ESATA instead.
To expand on Lurkers post
SATA is the most common hard drive connection used in most machines now a days.
eSATA is the external version of this. Some external hard drives, use this connection.
They are PREFERABLE for use, just for the case of that the TRANSFER speed across that type of connection, is much faster than USB normally.