Are you asking about when someone tags you in a photo on facebook?
If so then if you use the tag option it helps other people identify who you are in the picture. Such as if you have three people in a picture you always talk about them to one of your other friends and you have a picture of them, you tag all of those friends in that photo now the person that doesn't know them can now put a name to a face. It's helping with identifying people to other people along with people that do have cameras veruses people that don't. If one of your friends doesn't have a camera you can tag them in a photo and it will automatically go to their photos on facebook how ever if they choose not to have that photo on there they don't have to . There is a way to delete it off of your tagged photos