I have an access point with a USB port on it. It also has a console and ethernet port. What is the USB port used for? At first I thought it might be the port used for serial but rechecking using google that is the console port. It turns out the more modern routers can connect with other peripherals such as external hard disk and printer and even to a 3G dongle.
I think there might be exceptions such as with the Alfa AP121U access point. When you flash it with Pinneaple router you use the USB port.