Attenuator essentially reduces the "reach" of a scanner.
It would be helpful if you are in a metropolitan area and get a lot of interference from a bunch of transmitters in the area. Also if you listen to a simulcast system and see a lot of garble.
Probably in your application it would be if an agency 50 miles west uses the same frequencies as another agency 30 miles east. The attenuator could "tune out" that 50 mile west agency.
