You have a better chance to sell to them later. You may end up selling them an affiliate product and not your own. If you had not gathered mailing list that visitor's email address they would have been lost and never seen again, but since you have it, you have a shot at selling to them and keeping them later, as well as building trust in you that you know your business well enough for them to buy from you later.
Companies and organizations have spent many billions on e-mail and other Internet message delivery mechanisms over the past decade, often at the expense of more traditional marketing methods, like direct mail. But in some respects e-mail has mailing list failed to live up to its initial promise. And marketers who are turning back to tried-and-true methods like "snail mail" report excellent results, often better than e-mail.
According to a 2005 Direct Marketing Association (DMA) comprehensive study of marketing tools, e-mail produces the best return on mailing list investment and is the cheapest and fastest direct marketing tool. But only a fraction of the average company's prospective customers opt in to most rentable lists. And it can be against federal law to send commercial e-mails to people who have asked not to receive them.