The reason for this is that you show that you care about your work. It gives the publisher a great impression of you and increases your chances of being published. It shows that you care enough to send your very best writing. In short, you're putting your best foot forward.
Of course, you can just write something and submit it. But did you check if you showed, not told? Have you replaced the 'to-be' verbs with action? Did you write tight? Even if you did all that and more, you should still hire an editor.
I can hear you thinking, "Why should I hire an editor when publishers will assign me one?" We all, as authors, tend to fall in love with our words. An editor will catch those errors. Also, if you've decided on self-publishing, there is no publisher. Besides, as I said in the first paragraph, you want to put your best foot forward. This is like a job interview. The boss isn't going to hire an unknowledgeable and unkept looking person. On the same lines, a publisher isn't going to accept an error-ridden manuscript.
Where do you find an editor? Well, you can do a search on Google or your favorite search engine, You could even go to places like KidLit 411 or SCBWI to see what they have to offer. Or you can ask other authors in your writing group. This is also a great resource! They'll have great suggestions.
On that note, I'd like to offer my own suggestion. Janice Spina is great and her prices are very reasonable. For more information on Janice, just click on her name. This takes you to her website.