I'll chirp in with a titbit (ready to be shot down
)
CPL's replaced linear polarisers when SLR's came into vogue. If there is a strong light source (ie, the sun) at about 90 degrees to the lens surface then the uncontained light will cause metering problems as it's reflected off internal surfaces in the camera body...especially the mirror. CPL's are made to rotate as different lens and camera internals require subtle difference in the position for the optimum protection/correction.
Okay...fire now !
Bob