Salt was necessary in every sacrifice. Presumably its symbolic value was important. The key to understanding its symbolism lies in the phrase, the salt of the covenant with your God.
This suggests that the salt itself symbolized the worshippers' covenant with the Lord. In the Old Testament salt is referred to, within the context of a covenant, on two occasions (Numbers 18:19; 2 Chronicles 13:5). In Numbers 18:19 the Sinaitic Covenant is in view, while in 2 Chronicles 13:5 the Davidic Covenant is highlighted. In both instances a covenant of salt means it is synonymous with an eternal covenant.
13 Tu mettras du sel sur toutes tes offrandes; tu ne laisseras point ton offrande manquer de sel, signe de l'alliance de ton Dieu; sur toutes tes offrandes tu mettras du sel.