I'll try...
At idle, if your engine is turning relatively slowly, let’s say 1000 rpms, it gets little airflow through the intake. The throttle is closed, so very little fuel and air are being drawn in to the cylinders. This small amount of combustible mixture burns very quickly, so for maximum efficiency, the spark needs to start when the piston is very near top dead center.
The elevated head temperatures of an air cooled motor can sometimes cause pre-ignition, If this happens, the pressure from the ignited mixture will hit the piston while it is still coming up the cylinder and be wasted trying to shove the piston down before it reaches the end of it’s travel - this causes excessive stress on the piston, connecting rod and crankshaft.
Setting a higher idle speed increases the flow of cooling fuel and air through the intake, preventing this type of condition.