Aside from being a) very good target practice and b) good at scaring Soviet peasants, the Ratte seems to have no discenerable function in WWII (and was never made).
Perhaps in scenario where the Axis achieved peace with the Soviets, and thus had less supply problems, it could have function in a scenario where the Nazis had total air supremacy?
Let's be honest Hitler would probably build the thing anyway post 1945.