Posted By:
davidcyclebackMike, I agree in that the details from the caption can add value. If it's a face shot of Babe Ruth, one doesn't need a caption to know it's Babe Ruth. On the other hand, if it's the 1942 Army football team portrait, it's beneficial to know the players' name and that the photo was shot before the the big Army Navy game.
So a tag isn't always needed to authenticate a photo, but can add details that will raise the sales price-- especially, when the owner and buyer would know little about the image without the caption.