It is difficult to claim that any specific "discoveries" have been made in/about writing; however, there are a number of schools of thought about writing that help us to understand what writing is and does. These lessons can potentially be used for practical application beyond existing as novel literary devices.
One school of thought, structuralism, sought to explore what common structures exist across pieces of literature to make them "great works." These structures, theoretically, could be universally applied to pieces of writing to elevate them. One theorist, commonly described as drawing on structuralist ideas, is Joseph Campbell. Campbell developed the idea of the monomyth, suggesting that nearly every epic myth follows a series of predictable steps from beginning to end (a call to adventure, a refusal of the call, external guidance from a god or mentor figure, etc.). This structure of steps is used in many stories today, including recognizable films such as animated Disney movies and Star Wars, and it can be used as a heuristic or template for budding creative writers. Social worker and self-help guru Brene Brown has used this approach in business, having successful businesspeople tell their stories based on the steps of the monomyth to explain how they achieved their business goals. It continues to be used in different areas of social and educational research as a way to learn from others' experiences.
While it may be a stretch to call this a discovery per se, the monomyth is a way of understanding literature that can be used outside the contexts of the literature classroom, both to help creative writers and to situate people's stories.