This question is a based upon the figure of speech (simile) of comparing people to salt. Why did our Lord do this? He had just delivered The Sermon on the Mount, where He challenges us with the Beatitudes. So the thinking goes that as salt is used to enhance the flavor of even the blandest of foods, so we should enhance our world by imitating His call toward blessedness.
So we are called to be that salt, because if we don’t use our talents and capabilities to imitate Christ, then we end up corrupting that same mission, just as road salt expedites the rusting of cars and degradation of the roads and bridges. Jesus asks us to be an enhancement to those around us, not a corrupting agent.
In a similar way, just as the Romans salted the fields of their enemies, we have to salt the fields of our enemy, the devil. By doing good to others, preaching the Gospel, we affect the evil of the world of which we are surrounded.
Until next time…keep asking questions of Jesus.