Structure is beneficial to creative output in a number of ways. There are at least two types of structure, work processes and frameworks:
a) Work processes such as incremental production produce more output than a "do your best" approach. Writing four pages a day completes a words-on-paper first draft screenplay in one month. A "do your best" or "waiting for inspiration" approach can take months or years.
b) Work processes such as separating creative from critical thinking allow build up of large idea pools using creative thinking and reduction of those pools into feasible ideas using critical thinking.
c) Frameworks reduce complex problems into their component intellectual parts. For example, story structure can be reduced to three or four acts or The Hero With A Thousand Faces (Campbell, 1973). Frameworks increase output by reducing complex problems into smaller, more manageable problem solving exercises. In screenwriting, frameworks tell writer where to start, where to finish, what to write and what should be happening at a particular stage of story.
Additionally, a structured approach improves performance in a number of ways, including:
a) Simply being prolific improves performance. The single best creative product tends to appear at that point in career when creator is being most prolific. Experience refines knowledge and methodology towards optimal levels.