As a writer and an editor, I have to say that editing is very important! :)
A good editor can help: 1) Read the piece as your intended audience to flag questions, gaps, etc. 2) Review structure, flow, tone... all that stuff that you're probably too close to see objectively. 3) Identify language issues that affect your message or your communication of that message. 4) Clean up grammar, spelling, etc.
When you work with an editor also defines what the editor can do for you. You may only contact an editor at the end of the writing process. But, you can also work with an editor earlier to come up with solutions to sticky writing problems (two heads are often better than one).
I'm lucky to be in a writing group with other editor/writers. I get help thinking through plot ideas, character description, motivation. I guess what I'm saying is that an editor can be one of your greatest allies.
What makes a strong edit depends on the type of writing and your goals. For highly creative work, like poetry or fiction, it is critical to keep the writer's voice and intentions. For technical work, it is often important to replace the writer's voice with the company's voice but still keep the information and intentions. Identifying the goals of your writing and what you want out of the editing will help the edit pass go smoothly.
If your goal is to make your writing the best that it can be (for example, to send out to publishers), then I suggest the following: 1) Choose the editor carefully. Make sure he or she has experience in your field. Go with referrals if possible. 2) Leave plenty of time for editing, reviewing the edits, and any possible rewriting. 3) Clearly define up front what you expect and want. 4) Discuss your preferred working style. (Do you want the editor to rewrite sections with changes tracked? Or would you prefer any suggested rewrites are outside of the document?) 5) As much as possible, leave your ego at the door. (Feedback and revisions can be painful, but looking at them objectively and considering them leads to better writing. Even if you ultimately do not follow a suggestion, it is worth thinking it through. Sometimes it sparks a new great idea!)
I hope this helps!


