Unless your company has a very unusual website; the majority of visitors will leave it, without any kind of interaction.
Retargeting (Google prefer to call it Remarketing) tries to bring those people back by advertising your site on websites that they subsequently visit.
It is this cookie that allows the Retargeting service to track them and to serve the relevant online adverts through partnership with companies such as Facebook, Google, OpenX etc.
It is proving to a popular technique:
"The percentage of marketers that now spend over 50 percent of their digital ad budgets on retargeting doubled from 7 percent to 14 percent this year. That number is even higher in large companies; 24 percent of firms with over 1,000 employees say they spent at least half of their online ad budgets on retargeting."
But, as with everything, there are those who take the opposite view:
"Retargeting is based on the past. The clumsiness of retargeting is well-documented and we're assured that teams of scientists are working away to improve the algorithms -- these are the new media buyers, planners and creatives of the performance marketing age."