The salt acts as a separating agent by raising the relative volatility of the mixture and by breaking any azeotropes that may otherwise form.
The technique is first attested in writings on alcohol attributed to Jabir ibn Hayyan (9th c. CE).
In addition, the salt ions have a greater effect on the volatility of the mixture to be distilled than other liquid-separating agents.
[1] Commercial usage of salt-effect distillation includes adding magnesium nitrate to an aqueous solution of nitric acid to concentrate it further.
Calcium chloride is added to acetone-methanol and water-isopropanol mixtures in order to facilitate separation.