I reverse negatives in two situations.
The first thing a portrait client sees is a reversed proof. It is the face they see in the bathroom mirror. I sometimes get a response like "You're the only photographer who has ever got me the way I am. All the others seem to get me wrong somehow." The other portraits in the shoot are the right way round so that everyone else will recognise my subject properly.
The other flip involves self portraits when I wear a particular T-shirt on which the words "GUARANTEED NO DIGITAL" are stenciled backwards. By flipping the negative the positive shows street signage reversed, cars on the wrong side of the road, shirts buttons wrong, wrist watches on the right, and so on. In effect I create a Bizarro world in which everything is awry except the message on the T-shirt!