Both the author and the audience greatly benefit from the writing. Writing clarifies and structures thinking – helping the reader understand the points the author is making.
As a bit more context, I used to work at McKinsey and much of my job fit into two roles: (1) translating what employees were thinking into something executives could understand, and (2) making PowerPoint slides. In other words, I was often there because employees couldn't write well. But, I also found PowerPoint lacking – it's hard to get some of the more important points across (creating some confusion) because it doesn't allow for longer-form thought.
I've put a bunch of thoughts together on why writing is important and how we fix our education system to make people better writers. It's based on my experiences supporting tens of thousands of students on improving writing skills – https://bradsblog.com/2020/05/15/1-writing-is-the-most-impor....