I think they come with an earphone jack, this will allow you to just plug in an ipod & any control of songs will be via the ipod. With the additional aux/usb socket you can plug in your ipod & it'll work much the same as a CD auto changer would, plus you should be able to plug in a usb stick with mp3s on it & run it in the same way.
At least thats how I understand it.