Retargeting has one purpose, to bring back site visitors to convert them into customers. Since 98% of all site visitors will not act on their first visit, retargeting keeps the message visible and provides a path back to the site.
Visits to a site don't occur by accident, but rather because the consumer has been conditioned to think of an advertiser when considering a purchase of a product or service. It makes sense to direct the ad message to people who have already visited the site and who are interested in the product or service, and to do so on sites they visit in the future. Retargeting is an excellent way to continuously reach future customers who have previously demonstrated interest in the product or service.