Why did Stephenie Meyer write Bree Tanner?

Stephenie Meyer's novella "The Short Second Life of Bree Tanner," a vampire-side addition to her Twilight series, can be read online for the next four weeks, for free. And readers who want an offline version can also buy the hardcover.
Selling books, though, shouldn't be much of a concern for Meyer. She's earned almost as many devoted fans as Harry Potter, and her books -- individually and packaged as sets -- have enjoyed many, many, many weeks on bestseller lists.
So what compelled her to write the new novella? The story of the new vampires -- newborns, in her parlance -- as she explains to USA Today:

While I was writing Eclipse, there was a lot going on behind the scenes, of course, things Bella didn't know about. Because I was focused on Bella, I couldn't delve too deeply into the newborns' story, however, there was always in my mind a general idea of what they were up to. I had to think about that while I was pacing the plot: Bella is at this point, the newborns are doing this. To keep it all straight, eventually I made a calendar of the months of May and June — which is all Eclipse deals with — and wrote down on each square what was going on with Bella that day and what was happening in Seattle. So the story of the newborns was always a big part of the story of Eclipse. And it made me kind of sad that there was no way to express any of that in the book.

Maybe, as critic Lizzie Skurnick suggests at Salon, the story would have been better left unexpressed.

In the introduction to "The Short Second Life," Meyer tells us, "Writing Bree was the first time I'd stepped into the shoes of a narrator who was a 'real' vampire -- a hunter, a monster." Wanting to stretch beyond supernatural beings who endear themselves to us with their winning traits is a worthy goal, but for those accustomed to Cullens and Blacks, life as a real-life monster, I am sorry to report, seems dismal indeed....
Meyer's talents lie not in filling in her characters' backgrounds but in maintaining them in a constant state of exquisite torment. Intelligent and gifted, Bella, Jacob and Edward are intrinsically glamorous to readers. But Bree is PVT (poor vampire trash) and she knows it. Unqualified to even understand her situation, much less change it, she inspires empathy in the reader, but not a whole lot of anticipation.

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~Robstenfan