Why is it important to work on the technical craft of writing?

Because these are four completely distinct things:

1. What you intended to write

2. What you think you wrote

3. What you actually wrote

4. How the audience reads what you wrote

The craft of writing consists in uniting all four.

In good writing, all four match.

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By the way: a really good editor can take what’s on the page, understand what you meant to say, predict how the audience might actually understand it, and then show you why there’s a disconnect and help you fix it.