Once upon a time it was difficult to start the average family car. There was a special procedure in winter and a different method in summer.
You needed to pull out the Choke knob just so far and no further. You also needed to have a plan if this method failed to start the car. You needed to know when it was safe to push the choke in, because doing this too early could cause the car to stop in the middle of the road.
Worst of all, you would not wish the choke knob to spring back into place before you were done with it. The safest approach, and the universal one, was to pull the choke out, and fit a peg behind it to prevent any such problem.
It is nice to see these old traditions still upheld at classic car shows.