Non iPod mp3 players primarily. I have the iPod jack that the HU came wired with, located in the glove box because I can control that through the HU (which you prob knew). But if someone has a non-iPod mp3 player, they can use the Aux cable by plugging it into the headphone jack and you would have to control the music through the mp3 player itself. I had the Aux cable come through the top of the useless shelf, instead of the back, for a slightly cleaner look. With the GPS mount, which was added later, so it doesn