Purpose of salt: to cause the sacrifices to burn more perfectly and completely, ie with a distinctive bright flame -- the "divine fire". Fire destroys, but salt preserves. Fire = strife, ambition; salt = peace, purity. For salt, see Leviticus 2:13; Numbers 18:9. Salt symbolizes incorruption, in contrast to leaven/yeast. All temple sacrifices were salted: we are "living sacrifices" (Romans 12:1,2). Salt is a token of the covenant: Every sacrifice must be in bonds of covenant -- ie done in a proper spirit. Cp Colossians 4:16 with Ephesians 4:29.
"Purity is not just for the perfection of the individual life but that its influence is for the common good. The purpose of salt is to purify, but not to purify itself. Its purpose is to purify the material to which it is applied. It is intended to halt the spread of corruption in the area where it is at work. The influence is altogether antiseptic. Purity in the disciple is like the antiseptic influence of salt. The disciple's purity halts the spread of corruption in the little world where the disciple moves. That point is stressed by the word earth. It is not in heaven, nor in the world at large but 'Ye are the salt of the earth.' The pollution is of the earth, earthy: the standing ground where men corrupt themselves; on the level of the material, in the valley where the dirt collects, where the stench is bad and disease abounds.
"Salt bearers are not called upon to cleanse the universe, but that limited sphere of influence where every day they move. It is not a crusading march. It is not banner work. It is not with the backing of a great trumpet. The effect of salt is unobtrusive, like leaven in a whole lump: quiet, unseen but real. Disciples become salt because of what they are" (GD).
I hope you have found this helpful.