Why are cookies useful?

Some scenarios first:

• Alice is shopping at a particular web site that uses a shopping cart metaphor.  She puts items into a shopping cart by clicking a link or an "Add to Shopping Cart" button.  Cookies can be used to store the contents of Alice's shopping cart so that she can conveniently purchase a cart full of items rather than one item at a time.

• Bob fills out a web form with his name, address, and other information.  Cookies can be used to store this information so that the next time Bob visits the site, the information is automatically uploaded and he doesn't have to provide it again.  If the form contains sensitive information such as a credit card number or a mailing address, the cookies can be delivered over Security Sockets Layer, which encrypts the information as it travels between the client and server.

• Carl clicks around a web site that allows users to specify their interested fields.  Cookies can be used to store information about which categories he has chosen so that the next time Carl visits the site, only those chosen categories are displayed.