A Cookie is not a perfect mechanism, but they do make a lot of things possible that would be otherwise impossible. Here are some reasons why a cookie is imperfect.
- Computers used in public areas and many computers used in office environments, or at home, are shared by multiple people. You use one of these computers to make a purchase and the online store leaves a cookie on that public computer. It's possible that someone could later try to purchase something from the store using your account. Online stores usually post large warnings about this problem, but mistakes happen. For example, I had once used my girlfriend's computer to purchase something from Amazon. Later, she visited the Amazon site and clicked the "purchase" button again, not realizing that it created a second purchase of a book.
- A Cookie can get erased - If you have a problem with your browser and call for support, the first thing tech support will ask you to do is to erase all of the temporary Internet files. When you do that, you lose all of your cookie files. Now when you visit a site again, that site will think you are a new user and assign you a new cookie.
- Multiple machines - People often use more than one computer during the day. For example, I have a machine in the office, a machine at home and a laptop for travel. Any site I visit from these three machines will track me as three separate users. Recording a false number of unique visitors and/or buyers.