Are we talking about the USB in the armrest (for plugging in USB Memory Sticks with music on) or the one in the glovebox as micz4 seems to be talking about the armrest and the OP the glovebox...
If armrest, this (prior to MY2012/11) was optional and if fitted allowed you to play music on a USB device. If glovebox this is for importing Music and Navigation Routes and cannot be used to play music directly. The glovebox USB port is awkward to get at, but not impossible, having said that, I do not have the CD changer as I import all of my music to the hard disk for ease of playback.