This might have to do with our example and our righteous influence. The righteous people in Zarahemla postponed the destruction of that city. Or it might have to do with our genealogy work. Without genealogy, the whole earth would be utterly wasted at Christ's second coming. We can help preserve family values and morality.
Salt conducts electricity when placed in water.
Maybe we help conduct the power of God to mankind. Salt kills germs and helps wounds heal. Though sometimes it is painful, salt in wounds helps kill germs. Maybe we are to fight the germs in our society. Maybe we are to help purify and heal the people of this world.