Writing is thinking. A lot of unhashed inklings of ideas will become clear after writing about them.

It starts with the blank screen. The mind has to start thinking about the topic at hand just to come up with things to write. And as it starts generating, many of those ideas will need vetting.

Choosing what to include and what not to include is the most important part of writing, especially for writers who strive to be clear and concise.

A writer who aims to explain their ideas plainly and simply will vet many wrong directions they could've taken their piece in, many wrong words that don't quite convey what they're trying to say, and most importantly, many ideas that sound right at first but fall apart once you put them into words.

Just as science arrives at the truth by disproving hypotheses, deciding what not to write forces the writer to think more critically about their topic.