Why is brine added during the boiling process of soapmaking?

In the classical boiling process of soapmaking, brine is added to the kettle to separate the glycerin from the soap, a process known as "salting out".1

This separation occurs because soap is insoluble in a salt solution, while glycerin remains soluble. When the pasty boiling mass is treated with brine, it separates into two distinct layers:1
* Upper layer: A curdy mass of impure soap.1
* Lower layer: An aqueous salt solution containing the dissolved glycerin, which is referred to as spent lye.1

By extracting this spent lye from the bottom of the kettle, manufacturers can effectively remove the glycerin and subsequently treat the solution for its recovery.1