Why are they there?

Some people used it for a third party USB modem to play Tony Hawk 3 online. They aren't of so much concern now, but when the linux development kit and more online titles come out you will be able to do a lot of things a computer can. The development kit is shipping with a USB mouse, keyboard, hard drive, and ethernet/phoneline modem periphacals. You of course get the development suite and I believe the correct cable so you can hook it up to your PC monitor. The SONY site has it for preorder at 199.99 I believe.

I vaguely recall that a digital camera can be used with Monster Rancher 3 (to put pics on monsters' masks) - I'd imagine the camera would connect through the USB port.

Logitech's wheel uses the USB port, too.

My only use: the Sharkport, to connect the PS2 to my PC. That's how I got the NCAA 2002 roster update to my PS2.

Don't a few games (likely FPS) support a USB mouse and/or keyboard?

The Guncon 2 uses the USB port.

The headset for S.O.C.O.M. will be USB I believe.