The main intent was to pass emissions, the side benefit was lessening
run on/backfire.
There is always unused fuel in the system on shutdown.
With high combustion/exhaust system temps, one can experience a backfire.
Also, oxygen can enter the muffler/exhaust system through clamp joints, bad gaskets or degradation of the system.
This will also cause a "pop" or backfire, sometimes causing extensive muffler damage.
Most equipment manufactures will recommend a "normalizing" or cooling off
( idling) of equipment after hard usage, for a few minutes before shutting off.
Just common sense.
And I'm with Sam, debris in the cooling fins do no one good, best to clean.
