It's not an essential upgrade for 99% of racers/bashers however, it's a high propotion of the top on and off road racers (mainly in the UK) use them for getting more power/torque out of their engines.
The unit is programmed to change the ignition timing of the engine to improve performance - it advances the timing where normally the standard timing is retarted at higher engine speeds (rpm).
It displays the current rpm, current run time, peak rpm, total run time. To keep it legal for use at national and international race meetings the unit is powered via the ignition coil.
I found it helps with the clutch engagement point and gearing - the increase in engine power is minimal on a tuned engine but is noticable on a stock/standard engine.
Clive