Thunderbolt ports, like USB, also carry both power and data. So if it is used to connect the computer with the Thunderbolt port on the monitor, then there is no need to connect the computer with the upstream USB port of your monitor with a separate USB cable. Using Thunderbolt port on a monitor will automatically activate the downstream USB port(s) on most monitors and the computer can use the devices connected to the monitor, through the Thunderbolt connection.