Two benefit that I'm aware off are that it raises the temperature of the mash up to sparge temperatures, and decreases the viscosity of the mash. This is very useful when fly sparging, when the sparge water is added very slowly as the wort is being drained. In this case, without a mash out, the sparge temperature will probably not get above 160.
When I started doing a mash out (with a fly sparge), my efficiency increased from 75% to 85%.
On the rare occasions that I batch sparge, I also do a mash out. I know that adding 1g near boiling water is going to raise the temperature from mash temps to 168 - 170 so that each addition of sparge water can be at the same temperature. If I didn't mash out, I would have to calculate a different temperature for the first addition of sparge water.
-a.