debeer1-3, Exercise 2: Reasons for the order

Model answer:

Any three of: keeping wheat and rye for bread-makers by limiting brewers to barley; quality and safety, by stopping cheating and unpredictable additives; price control and tax revenue for the duke; asserting ducal authority with one uniform rule across the duchy.

Which is most convincing is a judgement. A common choice is the bread-grain reason, because grain shortages and hunger were a constant danger and the order's wording about barley fits it well. The quality motive is the one advertising prefers, but the evidence for it is weaker.