Have you every had a past thought after seeing something you almost purchased and felt disappointed that you didn’t take action when you had the chance? We all have and likely how the birth of re-targeting with display ads was derived.
Your website has many visitors, who arrive on their own luck or via a digital marketing channel you invest into, that leave the site before making a decision to buy. Unfortunately some of them never would have converted, and even more unfortunately many of them actually would buy, but just not today.