At the end of the film the audience and all the characters within the movie learn that Norman Bates's alternate mother personality is actually responsible for all the murders.
This mirror is placed there to show Marion as she makes the decision to use a fake name in the registry. Although the killer is not yet present we do get a glimpse of Marion from outside the curtain. Mirrors play a crucial part in reflecting characters' important decisions, as is shown when Marion trades-in her car.
The music in Psycho is 'non-diagetic' and is performed by an all-strings orchestra. The sequence ends with the masterful shot of Bates pushing Marion's car containing her body and the cash into a swamp. Seeing the shower scene today, several things stand out.
A pathological boy, with a telling hobby, a peephole, and personality to spare.
That it has been imitated often, never surpassed, is evidenced by 40 years of slasher flicks including its three variable from so-so to stinkbomb sequels, a TV movie spin-off and, especially, Gus Van Sant's re-make curio.
That's largely because of Hitchcock's artistry in two areas that are not as obvious: The setup of the Marion Crane story, and the relationship between Marion and Norman Anthony Perkins.
And the secret of Norman's mother is revealed. After the murders have been solved, there is an inexplicable scene during which a long-winded psychiatrist Simon Oakland lectures the assembled survivors on the causes of Norman's psychopathic behavior.