Every writer needs a second pair of eyes.

• Checks transitions between sentences and paragraphs. It is easy to know which way your writing is headed when you are the writer and know the material. It's not so easy for the reader who is new to the subject.
• Points out words that may have connotations you didn't intend. A word may mean one thing to five people and something slightly different to a sixth -- something very hard to catch without that "second pair of eyes."
• Asks you to clarify sentences or paragraphs that might be difficult for the reader to understand. It often helps when the editor has less background in the subject than the writer. He or she is then more at the level of your readers and will notice when something is not explained well enough.
• Catches misused words and grammar mistakes. There are many words that are commonly confused -- even by very good writers -- and grammar mistakes creep up on us all.
• Makes sure terminology is consistent. Do you write "web site" on one page and "website" on another?
• Catches spelling errors your word processor didn't flag. Even the best word processor can't tell when you mean "stationary" instead of "stationery" or "principle" instead of "principal."
• Advises you on stylistic choices. Should you use italics or quotation marks? How many dots in an ellipsis? (What is an ellipsis?!)
• Suggests ways to make your sentences sparkle. Learning to write is like learning to play a musical instrument or improving your golf game. It's all practice and learning new ways to write better, and working with an editor is a good way to learn.