Found this a few days ago. It solves a problem I didn't know I had: showing a map of all my photos locally (like Apple Photos), but not copying them or allowing modifications into its own library. I don't want sync, or the ability to edit them, just display and search them (and view and maybe edit metadata).
You just add a folder as a source and it'll create thumbnails, add metadata, do face recognition (if desired, using MacOS APIs), all locally.
One feature I would love: write the name of the recognized person to the metadata as a keyword (which I do manually now in Photo Mechanic).