not sure which model you have, but most of the Motorola (was General Instruments) have the usb port for 'future' use, most likely a keyboard or joystick, maybe even a printer.
Like the popular DCT5100 and up (DCT62xx) series actually have a docsis cable modem built in, so you could surf the internet right from your set top box, do two way interactive stuff, and so on.
I'm not sure any cable companies use these features now however....I don't think it's even enabled in most firmwares.
I've been hearing about 'interactive' television for years now...never really seen anything except like PPV or on demand video services...