OK, here is the real function of a turbo timer. Oil is supplied to a turbo for the bearing in the center shaft. When oil is hot and cools rapidly, it can "coke" the bearings, and wear out the bearing. To remedy this, the car should be allowed to cool down the oil slowly by allowing the car idle. Now, you could sit in the car and wait, or you can buy a Turbo Timer that automatically allows the car to idle, then shuts it off. For $75 - $80, it can save you longer time between rebuilding of the turbo. Plus, it is a great conversation peice like Terry said.
That is the Main function of the turbo timer. Some turbo timers have other features not related to this function, such as mine that shows the speed of the vehicle, can count 0-60, 0-100, 50-100 and some other times. Its just an added feature that has nothing to do with the turbo timer part.