She looked back because her heart was in Sodom. And Jesus said unto him, No man, having put his hand to the plough, and looking back, is fit for the kingdom of God. Although she had married Lot, whose only knowledge of GOD apparently came from Abram, she never became a Believer, even a carnal Believer. Two things I would have you answer. A pillar is upright and can be seen from a distance.
Salt is essential for life, and salt is used to purify and to preserve.
Knowledge of Lot's wife becoming a pillar of salt is an admonition to Believers to never look back fondly or with desire on their lives before becoming Believers. In this way, Lot's wife, as a pillar of salt, acts as a purifier and preservative for Believers. If one who believes himself to be a Believer is looking back and desiring the former life, then he better take inventory and check to see if he truly is a Believer. Just as if I'd never sinned. We will look at Lot's sons by his daughters at another time.