The USB adapter receives signals broadcast by a Wi-Fi router and then in turn transmits those signals to the computer into which it is plugged. This negates any need for plugs or bulky wireless cards by handling everything through signals.
Other wireless hardware requires installation in order to function. USB adapters are plug and play, meaning any software they need to get up and running installs automatically when they are slotted into a USB drive on the computer.
USB adapters can also be moved from computer to computer as desired. As long as they are within range of a wireless router they will provide a signal.