"Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity. ... If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master’s use, and prepared unto every good work." (New Testament, 2 Timothy 2:19;22)
LDS doctrine is repentance and sanctification focused. I think Paul's word's to Timothy beautifully encapsulate why this is so. If God's people hope to be "vessels of honor" sanctified for the Master's use, we must depart from iniquity. If we as God's people seek to carry the pure truth of God to the world, we need to clense the inner vessel or we will surely pollute the water.
When you visit a 3rd-world county, Water purity is a serious issue. You have to think about this issue constantly; not just when you are thirsty for a drink but also when you are washing your hands, brushing your teeth, and washing dishes. One drink of contaminated water can result in terrible consequences. But its not just the water that you need to think about but also the vessel or container. If you consume clean water in a contaminated container, its no better than drinking contaminated water.