In Chapter 4, Diamond is explaining why farming was beneficial to people and societies. He, of course, mentions the food value of various plants and animals. However, he then goes on (on page 90 and beyond in the paperback edition of the book) to talk about ways that plants can have economic benefits beyond their value as food.
The types of plants that you mention here are all good for the fact that they contain fibers that can be made into thread. The threads made from cotton and flax (which is made into linen) can be made into cloth that can be used to clothe people. Hemp can be used to make ropes and string to bind things together. In societies that previously had to rely on animal products for clothing and for binding, these would have been very economically beneficial.