Most of the premium routers now days have a USB port with it. Now you may wonder what is the significance of having a USB port?
A USB port is very useful in case you want to share data across the network. So in case one wants to share Information across to a group of people connected to the same WiFi network, he/she can easily do so by Inserting the USB in the USB port.
Another common use is to connect the USB port to a printer. So anyone in the network can print documents.
Normally routers have more than one usb port. They have a USB 3.0 port and USB 2.0 port. This extends the possibilities of what you would like to achieve.