From an author's perspective I have an innate understanding of plot and learned the craft of writing through long hours at the keyboard. From an editor's perspective I can see what's wrong in a manuscript and what may be required by the author to put it right.
Both heads can kick in when I'm writing or editing, sometimes in conflict with one another.
First drafting is very exciting, full of freedom and promise. Where is the story going, what will it lead to? But as Edgar Allan Poe says, “Nothing is more clear than that every plot, worth the name, must be elaborated to its dénouement before anything be attempted with the pen.” In other words, know the ending in advance, before you begin writing.
An author must know where the story is going but not necessarily how the characters will develop and what happens within the plot. A writer must have freedom for those "light bulb" moments, those wonderful flashes of insight and inspiration to work out how to get to the ending.
When editing I already know the ending. Editing involves a hypercritical view of the work, identifying what needs to go, what needs enhancing, where the author trips himself up with repetition, when her grammar and punctuation is off kilter. Dicing up someone else's baby can be tricky, therefore careful handling is required and encouraging the author to see their work in a different way.