The dull blade "frizzies" the stump and often kills the tree. Sharp blades prune it clean and it puts back out 4 or 5 inches up from the ground. Frizzie it right to the ground and usually the tree dies. Depends on what species of tree it is to a large extent. If your clipping pasture regularly you can sharpen your blades. Brush knocking back you need duller ones to kill the stumps and make the stumps frizzie so they won't puncture you tires the next mowing.