In ancient times salt could not be destroyed by fire or any other means. By adding salt to the offering, the worshipper was reminded of their eternal relationship with the Lord. This meant that the Lord would never forsake his people. At the same time, the worshipper was reminded of their duty to uphold the law.
13 You shall season all your grain offerings with salt. You shall not let the salt of the covenant with your God be missing from your grain offering; with all your offerings you shall offer salt.