I believe it is for back up, software updates, and adding profiles, like ones that Jingles makes or ones you make yourself.
I know on some other consoles, like the Show Designer line, you can use it to get a monitor out . This is done with a program and turns your computer/laptop screen into a screen for the SD. There is talk that USB displays will start popping up soon. I know High End is working on one for the Road Hog since it only has 3 USB ports on the console and no displays out. I know I am waiting for it for my Road Hog, though by then I might have a Full Boar or 3 if things keep going like they are
.
Don't know if this is the case for any other consoles besides the SD line though.