I think a big part of our role as salt of the earth (preservation analogy) is keeping good in the world. Everything good is found in God, and those who believers are his chosen vessels to give goodness/ good things to the world. God lives in us, works through us to bring hope to others. Christians preserve hope. I don't think we could do that if it wasn't for the seasoning aspect. If we didn't stand out, weren't different or tangy, who would know where to go? The more we look and act like the world, the harder it is to see the hope God is offering. I guess what I'm trying to say is without that salt, that distinctive element which sets us apart, no one would believe we (or God) can offer anything worth having, much less preserving.