Editors hear this a lot. Sometimes we say the same thing. Perhaps you have as well. There’s just one problem with the statement:
Everyone needs an editor.
Everyone.

One of the biggest challenges to good writing is the “curse of knowledge.” The curse of knowledge is when a writer knows what they1Yes, I used a singular “they” here—more on this in another post! mean, so they don’t bother explaining it.
You can have a perfectly grammatical paragraph, free from misspellings, typographical errors, and grammatical errors, and it can still be difficult to understand because the ideas are not adequately explained or the structure itself is unclear. Take a look at this example:
Van Gogh decided he would invite his friend Paul Gaugin to live with him at the Yellow House. Anxious because he was unsure whether Gaugin would accept, he arrived later that month and they began a residency together that was not without its difficulties.
The problem in this paragraph is in the second sentence: “…he arrived later that month.” Who arrived? Gaugin or Van Gogh? The author clearly understood that Gaugin arrived at the Yellow House because he knew Van Gogh was already there. But the structure is misleading. The dangling modifier “Anxious because he was unsure whether Gaugin would accept…” appears to be pointing at Gaugin because of the awkward syntax.
Having another set of eyes on a written text helps to remove the curse of knowledge. Because the editor doesn’t necessarily know what the author meant to say—indeed, sometimes it takes an editor several readings to figure it out!—the editor can clean up the text to make the content clearer.
Here’s another example:2Adapted from the Wikipedia entry
The word trebuchet comes from French. A trebuchet is a kind of catapult that uses a counterweight and a long arm to throw a projectile. Siege engines like catapults and trebuchets were often used in the Middle Ages until the widespread use of gunpowder. The design of a trebuchet is superior to that of a catapult because it can hurl projectiles much farther than a catapult can. The traction trebuchet, operated by men, is one kind of trebuchet that originated in China. The Christian and Muslim kingdoms around the Mediterranean developed the counterweight or counterpoint trebuchet. This trebuchet used a counterweight to swing the arm.
There is nothing grammatically wrong with this paragraph. It has no spelling errors or typographical mistakes. It has a lot of solid information. Nevertheless, it needs editing.
The connection from one sentence to the next is likely clear in the author’s mind, but the structure of the sentences doesn’t always make the flow clear to the reader. With editing, the paragraph becomes much clearer:
A trebuchet (pronounced “tre-byu-shay,” from the French), is a kind of catapult that uses a counterweight and a long arm to throw a projectile. Trebuchets were used in the Middle Ages as siege engines until the widespread use of gunpowder. They had a superior design to that of a catapult because they could hurl projectiles much farther than a catapult could. Trebuchets came in two main varieties: the traction trebuchet, developed in China, which used men to swing the arm; and the counterweight or counterpoint trebuchet, developed in the Christian and Muslim kingdoms around the Mediterranean, which used a counterweight to swing the arm.
This paragraph contains the same information as the first but is much easier to read. The ideas in this paragraph flow more readily into one another, lessening the burden on the reader.
The curse of knowledge is a genuine problem for writers—even those with good grammar and impeccable spelling. But it is a problem with a ready solution: good editing can overcome the curse of knowledge and make your writing clearer, stronger, and more effective.