It's useful for exploring your iPhoto library on other PCs or consumer electronic devices. It's also useful for backing up your iPhoto library (even to non-macs while retaining its structure.)
You can easily interact with your iPhoto collection using the Finder and command line utilities like find and rsync.
The file system is a MacFuse user mode filesystem that reads iPhoto's AlbumData.XML file.
Here's the current folder organization: