Subject: Why do tractors in a tractor-pull puposely spin their tires?

Slinding friction is less than static friction, so in most cases in a race or 
other event the racer wants his tires to spin as little as possible to maintain 
the most amount of friction and force on the ground.  However, in a tractor-
pull the tractors a built to purposely spin their tires.  What forces cause 
this to be more effecient than not spining the tires?  Is it simply the fact 
that the treads on the tire are slinging the dirt and propeling the tractor in 
the opposite direction, and this in combination with the sliding friction is 
greater than the static friction, or is there other forces at work?

Re: Why do tractors in a tractor-pull puposely spin their tires?