It supposedly prevents an air lock from forming in the upper radiator hose. It's mostly a "feel good" maneuver for people who don't want to go to the trouble to create a useful bypass to keep the coolant circulating past the cylinders before the thermostat opens.

Most good-quality thermostats have a jiggle pin built into them to accomplish the same thing without drilling any holes.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!