I read in the Canon manual to use the viewfinder cover for long exposures when you will not be looking through the viewfinder. Not covering the viewfinder is supposed to let in excess light and may overexpose the film. I wondered how it could do this because the viewfinder goes black during a shutter release because of the mirror. I was a little skeptical of this, but I see it written elsewhere. I thought the mirror would take care of this problem, but I guess not.
-SquirrelNuts