It has nothing to do with accuracy like the above posters said, but it can be handy for lengthening the range of your short range rifle. Lets say you set your scope 3" above the bore. Now, your bullet has to travel 3" higher to reach the crosshairs in your scope, thus lengthening your range by having the bullet travel higher, resulting in farther. It's really hard to explain, (typing it out, I mean) but my friends iPhone ballistic calculator proved the theory.
So raising your scope does not improve accuracy, it only lengthens the range. Hope this helps. Matthew