The rubber viewfinder cover blocks stray light from entering through the optical viewfinder.

It’s mainly useful when the camera is on a tripod for long exposures, self-timer shots, or any time your eye is not sealing the viewfinder. In normal handheld shooting, your eye usually blocks that light path.

If bright light is coming from behind the camera and the viewfinder is left uncovered, some stray light can enter the camera and potentially affect exposure or reduce image consistency, especially during long exposures. If the light behind the camera is weak, it often won’t matter much.

So: use the cover when the camera is not at your eye and there’s a chance of strong light hitting the viewfinder, particularly for long exposures.