Raru is right, it doesn't matter how amazing your story is or how much you develop your characters or how hard you work on your chapters if no one can understand it. Sure, sometimes little errors are alright, and people can still understand the story with them, but when they start piling on it gets a little difficult. A lot of people will put down a great story just because there are too many spelling errors or grammatical errors that make it difficult to understand.
Usually, authors will outline the general idea of what their story will contain, then they will go back and outline what happens in each chapter (although the exact contents of the chapter will often change as the writing progresses). Once they generally know what they want, they will start writing their stories. However, they will never just write a chapter and drop it completely. I know of several famous authors that have spent over a week agonizing over a single sentence and how to best structure it.
Granted, I understand that you have a life outside of writing unlike them, but taking the time to make sure things are spelled correctly and your grammar is good at the end of each chapter will save time later. Maybe you'll go back to the first chapter and be extremely confused as to what you initially meant by a sentence. Catch errors as they come up and stomp them to dust. Go back now and edit all of the errors and then continue with the rest of the story.
Maybe when you're going back over the story and correcting it, you'll realize something you could have said better or that there's a huge inconsistency in your writing somewhere. You don't want to be so far along in your story that it completely destroys it. What if that inconsistency completely throws off the rest of your story? You'll have to start from scratch. Better nip it in the bud early as they say than have to rewrite the entire thing right? After all, this chapter that you're busy writing could be made obsolete by that one inconsistency. You would have wasted all that time writing it just to scrap it and start over, not such a great use of your time.
As to the editor thing, they're there to catch the things you missed and look at the story from a fresh perspective to make sure everything makes sense. They are not there to edit every sentence for grammar and spelling errors. Yes, you might miss some things after going over your chapter again, and that's fine, that's what the editor is there for. But if you make them edit everything without even giving it a once-over to get the obvious stuff out of the way for them, you'll find yourself with an extremely unhappy editor, or no editor at all.
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