Each time you request a page, graphic, or other file from a Web site, a connection is established, the file is transferred, and the connection is terminated. It's not like logging on to your local network with a continuous connection. Consequently, it's not easy for the Web server to determine that the person requesting a page now is the same as the one who requested a page a minute ago. And for some applications, it's necessary to know that. Cookies are one way for Web site programmers to overcome this limitation.
Many sites use cookies to make things more convenient for visitors.
For example, some Web stores use cookies so that people who order frequently don't have to reenter their shipping addresses and other order information every time. Other sites use them to show what has changed since the last time a person visited. Popular reaction against cookies has caused some sites to stop using them. Although some concern may be warranted, and some people may want to take steps to control the use of cookies on their systems, there's no reason to panic and toss the cookies out with the wrapper. Giving up cookies entirely, just because you're worried about problems, many of which are imaginary, is an overreaction that will keep you from getting full use out of some Web sites.