Turbo timers are a nice way to give your turbo longer life. If you turn your engine off right after pushing it hard, there will be hot oil in the turbo. This can breakdown your turbo seals and do damage to the bearings. The easiest way to prevent this is by adding a turbo timer.
Turbo timers allow you to keep the engine running after you've removed the key from the ignition and walked away from the car. After a set time, the turbo timer will shut down the engine. This will circulate cool oil into the turbo before the engine shuts down saving those seals and bearings from deterioration. Turbo timers aren't a necessary item, but they sure are better than waiting in your car for 5 minutes while your turbo cools down after a sprited drive!