A WiFi adapter is for computers that don't have one built-in. Allowing the user to make use of the WiFi connectivity & convenience once inserted into the USB port.
A WiFi adapter and its uses
Before the time WiFi abilities were common people used USB WiFi adapters to connect their PC to the WiFi router. Available even today in case there is a laptop or PC that doesn't have built-in WiFi capabilities. Making a USB WiFi adapter an extremely useful device in that case.
The different types
Available under many brands there are versions that have an antenna that fits on to increase the range of distance over which it can catch WiFi signals or very compact ones that are almost flush with the USB port they are inserted into.
If you have a device that needs a simple solution for lack of WiFi capability a USB WiFi adapter can sort your woes, in most cases through just plug-n-play.