The responses I got as to why varied from liking to write own notes on index cards to having the subject expert create bullet points on slides to being comfortable speaking off the cuff.
A few days later, doing research on IdeaBank, I came across a description William G. McAdoo (1863-1941), American politician and leader of the Democratic opposition, gave about President Warren G. Harding's speeches:
"His speeches left the impression of an army of pompous phrases moving over the landscape in search of an idea; sometimes these meandering words would actually capture a straggling thought and bear it triumphantly, a prisoner in their midst, until it died of servitude and overwork.”
That, gentlemen, is why you need a speech writer.