The ticking clock helps propel the story forward by giving us a limit to how long our lead has to: get the guy (or girl), win the race, or save the world.
It is summarized well by Hitchcock who called it his BOMB THEORY - wherein he uses a bomb that the audience knows about but the characters in the play do not, in order to create tension and fear:
"In dramatizing this fear, Hitchcock employs a technique he calls the "Bomb Theory." This scenario runs as follows: Two men are sitting at a table discussing baseball. They talk for about five minutes, when suddenly, there is a huge explosion, which gives the audience a terrible shock, which lasts for about about fifteen seconds. According to Hitchcock's Bomb Theory, when the scene opens, you show the audience that there is a bomb under the table, which is set to go off in five minutes. While the men are sitting casually discussing baseball, the audience is squirming in their seats, thinking Don't sit there talking about baseball... there's a bomb under the table! Get rid of it! The audience is overwhelmed with the sense to warn the characters of the danger which they perceive, and which the characters are not aware of. Hitchcock's method transfers the menace from the screen to the minds of the audience, until it becomes unbearable - at which point there is a climax. An important footnote to this theory: You must never let the bomb go off and kill anybody. Otherwise, the audience will be very mad at you."
This can work without a physical bomb, although I actually kind of enjoyed MacGruber when I caught it on cable.
In a romantic comedy, the ticking clock is often the love interest's wedding or departure. In action movies, it's the pending death of someone who's been kidnapped. In awesome eighties comedies, it's the hip hop rec center that's going to be torn down. In THE LAKE EFFECT, it's the pending arrival of Celia's baby.
Anyway, right now, Luna's clock is a-ticking and I am off to get all my work done.
TICK TOCK! TICK TOCK!