The prevailing wisdom might be "not much," but Kent Anderson begs to differ. In a post on the Scholarly Kitchen site, Anderson lists 60 functions of the academic press, such as editing: "A subject-matter expert needs to learn how to be a good editor. This comes naturally enough to some, but others struggle with it, and a few never quite get it ... Some editing is cursory and done by outsourced editors with little domain expertise who just apply style guides."
