Covers are usually put on pickups to protect the coils from damage as on Fender style pickups and metal covers are used to help reduce unwanted electrical interference as on Gibson style humbuckers. Plastic covers don’t reduce hum or other electrical interference but add a desired cosmetic look and help reduce the wear and tear on a pickup. Some pickup covers are used to hold the components together as in a Gibson Firebird or Mini-humbucker. If you remove the cover, all the components can become loose and fall off the bottom plate.