If you live in a climate with a lot of rain, snow, sleet and hail, you may want to keep a couple of these in your glove box.  When your antenna isn't in the mount you can screw one of these plugs in an keep the "nasty's" from making a new home.  Also great for those who take their vehicle though the carwash or tend to back up a little too much when launching their boat (Ooops!).  Fits any 3/8"-24 threaded mount or accessory.