This has resulted in the emergence of a variety of techniques that let you identify and store data that pertains to a user. If the server assigns each browser a unique identifier, the server can identify repeated requests from the same browser. Browsers don't have to accept cookies, but because so many sites require them, most users don't completely shut them off. The first time a browser requests a page, the server assigns the browser a unique ID and stores it in a client-side cookie. Subsequent requests contain this cookie and the server knows they originated from the same browser.
Session management is important in many cases.
For example, when a customer fills a shopping cart, you'll want to remember the contents of that cart the entire time the customer visits the site. However, session management goes only so far. A browser can arbitrarily erase cookies whenever it wants to do so, and cookies eventually expire. It would be unusual to find a session that lasts for days or even hours. Also, cookies belong to a particular browser on a particular computer.