The problem with stateless interaction is that the Web server hasn't any idea of what's going on with you from page to page. If the server is allowed to store facts on your computer, in a place and format that it recognizes, then it can keep track of what you're doing on its entire site, and you can have a meaningful experience. E-commerce would be dreadful without cookies, since the computer handling your online purchases would be clueless at checkout about who you are and what you clicked among its products. Cookies are also used to govern advertising, so that visitors don't see the same ads over and over. This is valuable to the advertiser and site owner, but does not enrich the surfing mind.
Internet sites can share cookie information.
That's a mouthful. Be aware that nefarious Web operators can and do misuse cookies. Dropping by a rotten site gives that server the opportunity to write a program on your hard drive, which can execute later, forcing your computer to do ghastly things. A friend who had her browser set to accept all cookies clicked on a deceiving porn site link. To peek at a cookie, open it with a text processor.