What does the Crankshaft Position Sensor do?

[ScottStu]

I had to replace the Crankshaft Position Sensor last week do to it failing and turning off the engine. But what are all the things that it controls?
I ask this because I have owned my 98 Boxster since last May and it always made a little "rattle" at start-up but since replacing the sensor it's as smooth as silk and runs even better.