Table salt acts as a thickener in cleansing formulas created with ionic surfactants. Basically turns a cleansing formula into a nice gel texture. 

The science: the ions screen the electrostatic repulsion between head groups of the surfactant that support the formation of long shaped micelles, which entangle and form a gel. Too much salt though, causes a salting out and the formula becomes runny again. It can also work as an emulsion stabilizer in water in oil emulsions.