I would say that the main benefit would be a feature completness, meaning that in a rare situatuion a face detection algorithm misses a face the user is still be able to fix this manually.
I have a group picture with ~30 people on it. I can mark all of them on the photo except for two and it’s kind of annoying :( Of course, I can add the photo to their personal albums as you’ve suggested, but its not the same - when a face is properly marked, anyone pointing on it on the photo can immediately see the person’s name. In addition, selecting an avatar in the details pane highlights the person’s location on a photo. These are nice features that won’t work when using a ‘personal album’ functionality.
I agree that this is a kind of an edge case, since the face detection algorithm works surprisingly well. In my collection, only two out of ~150 photos has face detection issues. Still, i think this would be a nice addition that allows a user to control the situation instead of being dependent on algorithm performance.