It makes the art more "authentic" (if I can use that loaded term) if you have a story or explanation that goes with it. Or, it makes the are more believable. It shows the thoughts of the artist, otherwise, the viewer is left with control of the creation process. Or that is just my view. There are so many theories about who actually creates meaning in art, the viewer, the artist, the system in which the art is produced, etc. etc.