Photo Tags showing only in Library and not in the original folder

I was under the impression that an entry in the Photo library was a "pointer" to the original file in its folder.
Why then does a tag added in the "Pictures library" not show in the file in its original folder.
I thought that the advantage of tagging was that the tag went with the photo when transferred to anywhere else.
I have a folder "included" in my Pictures library - on another drive. Tags are there when I go in to the Pictures library but NOT when I go to the drive itself.