No matter how good of a writer you are, at one point you will need an editor. Why? Because you can't expect your brain to find the mistakes that it made up itself. Makes sense, right?
The most common things that we don't recognize in our own work are:
switching from past tense to present tense in the same text
leaving out important details
simple typos and punctuation/spelling mistakes
Plus, getting feedback from fresh eyes will give you new ideas, insights and tools to dot the i's!