What is the USB plug on the Mini-Display Port to Dual-DVI adapter used for? http://store.apple.com/us/product/MB571Z/A/mini-displayport-to-dual-link-dvi-adapter

What is the USB plug used for?
The Dual Link DVI connector is an active adapter, and requires a power source (which Mini-DP cannot supply). It uses the USB as a source of power to power the adapter.