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 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.