Digital hoarding isn't just backing up your family photos; it's compulsively finding, downloading and storing all sorts of content that you have no real use for (movies, music, TV shows, books, software, etc. -- usually in violation of copyright).
Most people enjoy commemorating a special moment with a photo, and being able to easily relive that memory a few years later. Smart digital technology makes this cheap and easy. Sure, having the camera out all the time is compulsive. Going through and manually editing, tagging and deleting photos is compulsive too. Most people do neither, but they still want to keep their photo memories safe.