So why do advertisers want to use cookies in this way?

Third-party ad serving companies provide useful, cost-effective services to websites that cannot manage advertising campaigns in-house. Advertisers like cookies to be used in the delivery of campaigns because they enable the following to happen without any personal information being collected:

• Cookies limit the number of times that an advertisement is shown. This is particularly useful if pop-up ads are used, as cookies can ensure the same browser is not shown the same pop-up over and over again.
• Enable a sequence of advertisements to be shown in the correct order.
• Calculate how many unique web visitors have been shown an advertisement.
• Calculate how many unique people visited a site as a direct response to an advertisement.