Demographic information is especially valuable, so some sites ask about the age, gender, and income level of their users. The sales data ecommerce sites collect is also usually integrated into the site's user knowledge base. Some companies leverage cookies even more. But you could be wrong. As was previously mentioned, advertising is an important source of revenue on the Web today.
Many sites carry advertisements in cooperation with companies who specialize in placing ads.
Many users may not have realized that the contents of a web page can be assembled from multiple servers, and that advertisements are often delivered directly from an ad company's server, rather than from the server that provides the rest of the page. This gives the advertising company the opportunity to set its own cookie and keep data about which ads have been served to a particular user visiting many sites. The advertising company now has the potential to assemble information from many sites to create a more complete dataset about an individual. If two web decided to partner with an advertising firm, the three companies could choose to share information about the users' demographic data, interests, and activities online. They may have information about purchasing habits of users if they sell products or services.