Farmall Cub

What is the purpose of the water jacket plate on my engine other than to keep the block from freezing and cracking?

I don't understand what you mean by "water jacket plate"

The plate on the side of the engine block. Looks like the the valve adj. cover plate on the cub engine, but it is called the water jacket cover on the " M" and " H".

Not familar with M or H but I am sure some one will be.

Our M has the same plate. Possibly for sand removal in casting ?
My Guess only

Back in the days when H and M were new tractors, and worked hard for their living, permanent antifreeze was very expensive. In the summer time most framers ran water that was dipped from a well or the creek, and in the winter alcohol. The plate made cleaning the accumulated crud out of the engine easier. Old timers used a lot of tricks to get by. Draining the radiator at night, and not using the tractor till above freezing the next day, then refilling with water, covering radiator in the morning so the fan would not suck in enough air to cool it below freezing, sometimes even starting the tractor before filling (combined with covering the radiator), so there was enough heat to keep it from freezing.