The most basic use of a speed sensor is to provide information about speed to the speedometer. This information is also used by the cruise control system and some measure engine revolutions per minute (rpm).

They can be located in the wheel hubs, you have wheel hub bearings, or they may be located on the axle.

I have a 2001 Chevy Blazer that has bearing hubs on the front and both hubs have the speed sensor. But also the ABS (if you have that) works with the sensors since each wheel can spin at different speeds therefore having to adjust braking via electronic (computer).