which soap?
salt is used to remove glycerin. in soaps made with potash (a lye of sorts from ashes in the old day which was really a POTassium product) the resulting soap was soft (we use potassium hydroxide to make liquid soap) so the soap was boiled with salt to swap out the potassium for sodium in the soap and result in a harder product. it may still be done for some reason today as well.