SASS is backed by Ruby, so you can take advantage of all of Ruby's power, including reading from the filesystem.

For instance, combined with Compass, you can read image sizes and automatically pull in width and height for a class. Or you can more easily make sprite images via a mixin that knows the size of the image and how many rows/columns you have.

I've been using bootstrap-sass since it was targeting bootstrap 2.3. They've done a great job of keeping up with bootstrap development, so it's great to see them officially supported. It wouldn't shock me at all if Bootstrap eventually moved over to SASS officially in the future.