Similar to UMDGen for PSP, it allows you to edit, rebuild, and create new images (in the proper PS3 ISO structure format) so that they will actually work with the PS3 once a method to play PS3 back-ups is publically available. This is useful for those who intend to make their own PS3 "rips" to save space- either to fit on a smaller disc or on the HDD, etc. There are several other tools/features that will be included in BRDGen, and as the PS3 scene grows and needs arise it will continue to be updated accordingly.
One suggestion I had was a PS3 SFO viewer/editor (although I'm not sure if that will be in an initial release or not, it was just a suggestion). If anyone else here has spent any time examining PS3 images thus far, feel free to add your suggestions here for BRDGen upcoming features and so on...