Editing is an essential part of writing, and any author will tell you the same. But don't get bogged down in the details of grammar and punctuation and forget to look at the bigger picture. When you're editing, you always have to check for gaps in the story.
Plot Holes and Other Errors
You're the author, so you know how the story ends. You know who the characters are and what's going to happen. And when you're writing, it's common to be focused on getting to the end. It's only natural for gaps to appear in any first draft.
But you've got to fill in those gaps when you go back and do the editing. Remember that the reader doesn't understand the characters the way you do, and they don't know how the story is supposed to end. You have to fill in the blanks for them. If your heroine is supposed to be brave and confident, write a scene that shows her acting this way. If the reader is supposed to care about a friendship between two people, write a scene that illustrates their closeness.
When you're editing, always be looking for the gaps. There's a good way to find them: ask questions. As you're reading, pay attention to any questions that appear in your head (such as, why are the characters in the desert? or do I know this guy Chris who's talking right now?). If you have questions, the readers will definitely have questions. Ask yourself questions as you read along. Do you understand why the character is doing this? Do you know why the character said no to Tom and yes to Jimmy? Are the motives and explanations part of the story...or are they just in your mind?
Fill in all the gaps in the story to create a much smoother read. If you don't catch the gaps while you edit...when will you?