Like baffles said, for their audio output. I have like 10 of these cables lying around. MP3 player, old razr's, garmin/magellan gps, they are all the 4 pin mini usb cable.
The details/pinout for the EMU (Enhanced Mini USB) connector are in this thread, post # 10...
http://forum.ppcgeeks.com/showthread.php?p=233792