They can be useful for a lot of detail type construction--latice windows and doors, broadway flats where keystones would not be an option, etc. It's faster than lap joints, and holds fairly well.
and it was handy, but there were also plenty of other ways to attach them without screws being seen and I would just as easily chose them over drilling pocket holes. Wasn't really worth it if you ask me. Though like Footer said, as a furniture maker I've wanted one of those jigs for awhile and would really put it to use there.