There are several types of editors, but no matter where you are in your writing career, you will need an editor.
No way can an author see or find all the typos, mis-matched words and plot points.
In an even bigger concept, someone (experienced) needs to read a book-in-progress. Is the theme being followed? Are there unnecessary scenes? Are there scenes that are hinted at, or alluded to that should be included?
Where are the weak spots? What are the book’s strengths?
The two main editors a writer working on a book are:
Copy editors who look for typos, checks spelling and looks for inconsistencies. A proofreader looks more for the first two items.
A developmental editor is much more involved in the story. This person works hand-in-hand with an author. Because cost and time are variables that dictate the depth that a development editor. The DE can suggest new scenes, cut others, ask for rewrites to get the book to adhere to standards of the genre.
Both editors are necessary. I would suggest that an author schedule and look into hiring both of these editors somewhere along the way in your writing journey. Many editors will offer to edit sample pages and you can determine whether you can work with this person.
Editors are as important as a word processor. Use them!