Jesus chose her as an stark example warning people against an attachment to ‘the old life’ (Luke 17:28-32). Genesis 19:15-25 tells how Lot was led urgently out of Sodom by the angels because it was to be destroyed. The angels told them not to look back or stop (Genesis 19:17). Lot’s wife went with him, but as they reached the place of safety and Sodom was destroyed by God, she looked back. Given how Jesus uses her example, it is very likely that her ‘looking back’ was both physical and symbolic: rather than looking forward to the new life she was receiving from God, she worried instead about what she was losing, even though what she was loosing was life in a sinful, godless place. God burned Sodom with fire from heaven and Lot’s wife was looking back at Sodom; God judged her behaviour and attitude and turned her into a pillar of salt.
Jesus warns us not to follow her example so that we will not follow in her punishment (Luke 17:32). Look forward to God’s reward – people who look back are not fit for it (Luke 9:62).