Hans Perk has posted a clip from Quark and the Highway Robber from the '80's. The great thing about this clip is that the click track is audible, and you can watch the animation and see how it relates to a musical beat.
For those who don't know, a director (with or without a musical director) will set a tempo for a sequence. In the case of the above clip, Hans set a 12 beat (a beat every 12 frames) for the start of the sequence and switched to a 10 beat as the action heated up.
By laying down the beat, the director can make sure than actions take place in such a way that when the music is added later, the action will work with the music. The musical director composes music (or takes music out of a library) that is timed to the same beat that the director has specified. When recording, the musicians will hear the click track to make sure that the music is played at the proper tempo.
Carl Stalling, who composed for Disney, Iwerks, Van Beuren, and was the major musical influence at Warner Bros. is credited with the invention of the click track.
This Quark clip is very different from cartoons from the early 1930's, where the visuals tried to hit just about every beat. The Quark clip shows that working to a beat is not a straightjacket; it's a convenience. It provides enough structure to give the director a way to time action coherently and guarantees that the music track will fit the action tightly.
This is a useful tool even within the budget constraints of a TV series that is going to build a library of music cues rather than use original music for every episode. If the director and composer plan things well enough, the director can work to a beat with the knowledge that there's an appropriate piece of music to accompany the action. This also speeds up the creation of the music tracks, as the music librarian can go straight to the appropriate piece of music.
If you're a director, an aspiring director or a student, the clip is worth looking at.