I'll add some more to that:
1. Mr. Diesel, who invented the Diesel engine, designed it to run on hemp seed oil. When asked why, he basically indicated, "Duh, it's only the cheapest most abundant oil on the planet".
Rural oil lamps used to run on hemp seed oil, before the Harrison Tax Stamp Act of 1937 outlawed (without the proper stamp which the government wouldn't let you have!) the interstate commerce of hemp products. This is partially responsible for the near extinction of whales, as whale oil was the next best thing to hemp seed oil for lamps.
Guess who were major supporters of the Harrison Tax Stamp Act? Not the AMA (American Medical Association), they were against the HTSA. But the petroleum industry (anyone can grow hemp, few can afford oil derricks, thus "Monopoly,yeah!"), the distillery industry, and the corn growers (who supply the distillery industry).
2. 10000 acres of hemp produces 2000 times the paper as 10000 acres of trees, and it is better quality paper. And 6 months later, you can grow it again. How long does it take to regrow 10000 acres of trees?
3. Hemp fiber is longer lasting (no planned obsolescence) and much more comfortable than cotton.
4. The bird seed industry was so aghast at the Harrison Tax Stamp Act that they got permission to continue using hemp seeds in bird food, although they have to bleach the poor seeds to make sure nobody gets high off of them (including the birds?)!
5. You're right. Iowa farmers were exempt from serving in WWII if they promised to grow hemp.
6. Also, despite the HTSA of 1937, 4-H organizations continued to promote hemp plot projects for kids into the early '40's.