A lot of Chrome apps are just an excuse to have a different sort of bookmark in the new tab page. They're just web pages that have been made into a Chrome app and labeled as such, despite not doing anything more than any other web page out there.
But there are also Chrome apps that, despite being web apps built on web technologies, run locally and get access to features regular web pages can't, they're called packaged apps.
They can now add the MediaGalleries API to the list of features they get access to. The MediaGalleries API, as the name suggests, enables Chrome apps to access local media content, i.e. music, photos, videos and so on.
The common media folders for the platform will be scanned by default, but users can add more locations as well. All of this requires user permission, of course.
For now, this is read-only access, but it should be enough to build a photo management app or even a music player for example. Support for writing files as well as creating new ones is in the works.