It depends on the application. They can be used with and without a MAF (Mass Air Flow Sensor)
Without a MAF
it's used alongside the engine RPM and throttle opening to determine the airflow into the engine, usually in petrol engines.
They are sometimes used with MAFs as well, especially on turbo engines where they are a measure of boost pressure, at least in diesels with no throttle.
They can also be used as a cross check to see if other signals are sensible.