Hmmm... If I knew I would have posted - I believe it is to make the soap 'harder' and to mop up inpurities, water and glycerine. I wasn't 100% sure which is why I didn't commint to an explanation.
Sorry if my sense of humour is not to your liking - perhaps you can use the salt to sprinkle on my only Scooby Snack which has now been eaten...