By adding a user in Created By, you are giving them credit as the key artist or creator of the piece. If you add a user under With, this implied that the user is associated with this photo. This means that they were involved in that piece of work, project or activity, but may not necessarily be visible in the photo.
Both face tags and With tags will be displayed in the same location, so you can see everyone who is a part of the photo. Face tags will be displayed as green, while With tags will be grey, as they have no face associated with them in the photo.