Like FunkyBoss said, they're increbily useful in tuning a vehicle. However unless you have some experience in tuning and/or have a power adder of one type or another, I would not worry about spending the money on one. The good ones are fairly pricey not very useful unless you plan on tuning the car yourself.
"Posterity! You will never know how much it cost the present Generation to preserve your Freedom! I hope you will make good use of it. If you do not, I shall repent in Heaven, that I ever took half the Pains to preserve it."