A good editor will clear away the literary clutter from your manuscript by tightening up your plot, concentrating your voice and ensuring your structure is as good as it can be; and if you think that your work is so good that it doesn’t need any editing at all, think again. Editing always improves a book when it’s done well, and it’s a much more subtle and complex process than most people realise. It is a real collaboration between the editor(s) and the writer: while the editor will suggest changes it’s the writer who is ultimately responsible for implementing them.
Writers usually only pay for editing if they’re self-publishing or vanity publishing: mainstream publishers employ their own editors and do not charge their writers for this service: it’s part of what they do as publishers, like designing, printing and marketing the books.
However, even though publishers edit the books they publish, writers should still work on their texts to make them as perfect as they possibly can before they submit them anywhere: it shows that they’re serious about their work, that they’re capable of producing polished and convincing texts, and that they’re going to be easy to work with—and it gives them the edge over other less-conscientious writers. Some writers use editorial consultants to help get their books up to scratch before submission, but such services can be expensive and of questionable value. There are some great editorial agencies out there but there are also some dreadful ones: if you’re considering taking this route it’s important that you research the options very carefully before you hand over your work or your cash.