There may be here seen a propriety in our Lord's language which may be gathered by considering the Apostle's office, and the nature of salt. This, used as it is by men for almost every purpose, preserves from decay those bodies which are sprinkled with it; and in this, as well as in every sense of its flavour as a condiment, the parallel is most exact.
A few more parallels:
Properly used, salt enhances the flavor of food, while too much salt ruins a dish. So salt is a good symbol of grace perfecting nature.
Salt is extremely valuable, but its value lies in its being poured out. Salt never used to season or preserve food, salt horded, might as well be savourless salt trodden underfoot. Salt is a necessity, yet it is important only for its effects on other things.
Reportedly, rock salt really does lose its savour if exposed to the elements. So, too, will the one sent by the Lord if he leaves himself exposed to the elements of the world rather than applying himself to its seasoning and preservation.
The disciples of Jesus are salt from the earth and salt for the earth. Jesus tells them this, not to praise them, but to inform them of what discipleship means -- and also to warn them that, if they give up on their discipleship, they will be worse off than before.