No duh. The universe of prior visitors to your Web site contains a healthy population of existing customers. Sometimes these folks need a reminder or an extension of value to bring them back to your Web site. If you're tracking conversions through your ad server, you'll no doubt see a nice bump in purchases in addition to clicks and click-through rate.
Now, imagine what a remessaging campaign could do if it was designed specifically for this audience. Different messages can be given to different sub-audiences based on past purchase behavior. Apply some basic database marketing techniques, just as you might in the direct-mail world. Visitors who did not purchase could receive a message that would promote switching behavior. Loyal purchasers could receive something that insulates them against poaching by the competition. You can even bucket different groups of purchasers to get a handle on loyalty, frequency and volume of purchasing.
The technology that allows us to serve different messages to different types of customers is not new, but it is underutilized. I see a lot of online ad campaigns that treat all people exactly the same--as if each recipient of an ad is someone who is unfamiliar with the brand and has never visited the Web site before. In some cases, this can have a detrimental effect on an existing customer base. ("What do you mean, this product is on sale? I just bought one last week! You should know that!")
If your campaign can recognize existing customers, though, that detrimental effect can be reversed. Your customers will be happier to be recognized as such.
If you sell things on your Web site and haven't tried this tactic yet, I would recommend it highly. Often, marketers wonder why it's seemingly difficult to cultivate loyalty. One of the reasons is that people like to be recognized for their loyalty, and we often treat loyal customers as if we don't recognize them.