Hitler spent time outside of speeches working on his body language. Those sorts of grandiose motions can be used to reinforce a point or sentiment, particularly when addressing a large crowd where the people in back can't see you very well.

Traditional ventriloquist dummies had such odd, pronounced features so that people in the back of vaudeville theaters could make out their faces. It's the same principle.

Roman orators had traditional hand gestures that they used to emphasize the points they made. They fell under the "actio" portion of traditional rhetoric.