I've always thought of it as (bad analogy coming up) the equivilent of a gearbox in a car, but when I've used it to explain to other people they seem to have got the idea.
A car has a gearbox, you pull away in 1st gear. In a plane you take off with prop set to full fine. In a car when you get to your desired speed you take your foot off the gas and knock it into a higher gear. In a plane you pull the throttle back and course the prop out. And don't forget you want it full fine again when you're coming into land - helps you get the speed off, and well, you know what happens when you try to pull away in a car with 4th gear selected... don't want that kind of thing when you're going around!