From a privacy standpoint, it ensures no one can break into the system to access your information. From a security standpoint, it also ensures that no one can steal your checking account number from the Internet. In the original design for the Web, there was no way for a Web Site to know whether one page request was connected to another. For most sites, this is fine. In a news site, for example, each article is independent of the others.
Visitors see what they want, then leave.
But for those of us doing Internet commerce, this is a real problem. We want people to be able to select several products before purchasing them. Internet vendors want the selection process and the checkout process to span more than one page. In other words, we need a way to keep track of Web Site sessions rather than merely indentifying single page requests. Cookies, which are small pieces of information stored on your computer, help do just that.