The question of who needs a professional editor is one I've come across a number of times. I decided it would be beneficial to post the answer here.
It's important to note that in this instance I'm referring to a WRITING editor. Not a music or video editor.
Firstly, a preamble: What is an editor? An editor is more than someone who simply checks your grammar. An editor will make suggestions regarding content (when applicable) and style. He or she will check your formatting and lay the document out for publishing whether it be online, as a thesis or scholarly paper, a magazine article, journal, etc
Who needs an editor?
Anyone writing a document for publication - students, authors, business people.
The reason for this is simple. Even though English may be your first language, it's very hard to spot your own mistakes. And a published document is a representation of you and / or your employer. A published mistake is a slur on your or your employer's reputation. It's no discredit to you to admit you've used an editor - in fact, anyone who knows this will appreciate your professionalism.
Someone who is writing in a language that is not their first language.
It's important to ask a first-language speaker to assist you to check that you've got all the nuances and idiosyncrasies correct.
Someone who doesn't consider language or writing to be their strength.
We all have different strengths - I have to hire a plumber or a mechanic, I just can't do those things. Therefore, if you are not confident in your language skills, using them to market your business might not be the best idea. A recent example I've seen is someone advertising Christmas signs. The signs say (for example) "Christmas at the Smith's". It makes me want to ask "what smith? The blacksmith?
Have a look at my new website, Check My Writing. Should you still have questions, or feel that YOU need an editor, contact me.