Perhaps a clearer example may be found in sports coaches.

The main reason that coaches are important (not just in sports) is because of blind spots - i.e., things that are outside of a person's direct perceptual awareness.

Think of the Dunning-Kreuger effect: if you can't perceive it, you can't improve it.

(This is also why publications have editors; if a writer could perceive the errors in their work, they could fix them themselves.)