I haven't a clue about what it's for. But, knowning what a USB port can do, it's most likely a DC power output for some device. USB ports have +5 volt DC (current limited to 100mA or up to 500mA) power on one pin and a ground return on another pin. The other two wires are for data communication only. No analog signals at all. Plus the data communication must conform to the USB standard.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70