Here's a fun sampling of a click track in use in the original production of Dreamgirls. The first link is the live orchestra and you can faintly hear the click track that was being sent to the conductor. The second link is the prerecorded vocals. The third link is the two combined.
I remember during a performance of Dance of the Vampires when the click track stopped working mid production number. It went from a very grand lush sound to a rehearsal piano in an instant. That was when I realized just how important click tracks are.