Sadly, not everyone who visits your site converts into a customer. Some people arrive there by accident, some realise you don’t offer what they need, some are researching on behalf of someone else, some go with *gasp* a competitor, while others just get distracted and put off the purchase for another time. Those last two categories hurt – the could have been sales. The good news is that no one who has visited your site and then disappeared into the aether has truly gone forever; we can follow them. What I really mean is that the cookies or pixels we place on our site can remember who visited and give that data to Google to allow us to show them our ads again.