The USB adaptor I assume came with the router and it is useful if the PC you want to connect does not have an existing wireless card. Most laptops these days already have a wireless card installed so they don't need the adaptor, but desktops rarely have a wireless card and relay on connection to the router by ethernet cable. In this case you could use the adaptor for more flexibility in where you locate the PC relative to the router.
In your set up sounds like you have no need for the USB adaptor unless to replace your cable connection for PC. Keep it as a spare in case you need it in the future.