Presumably a favor from the authors for those of us who wanted a little more closure to Filip's story.
I had heard there was supposed to be a story in it called Memory's Legion, but checked the fan wiki when there wasn't one, and apparently that story was renamed Sins of our Fathers because the publisher didn't want to create confusion by having both the stand-alone e-book novella and the print collection be called Memory's Legion.
The book also includes a page or two of author commentary after each story, which I found pretty neat. I've only read Drive so far, not really anything I hadn't seen before since it was adapted into a few flashback scenes for the TV show.