Not trying to argue, but thought it was interesting enough to ask why you do it. I know now you don't know why.
Don't worry, you are not unique. Lots of people do things all the time with out knowing why.
It was called temp treatment and normally you run engine to optimal operational temp and keep it there for a bit, then cool it of.
This was the way to do it on old engines that was not assembled with heat treated parts. Usually done at factory before it went thru final inspection and got out to be sold. Piston, rings, sleve, bearings, etc. today has this done before assembly.
Racing engins and other top performing , high out put or heavily modified engines are still temp treated.
When I worked on racing cars we did it to them after each teardown/race.