Sessional Cookies vs Sessional Storage vs Variables

If we can just store information in variables why do we need a sessional cookie or sessional storage?

 

You most likely would only store a session ID in a cookie. This will be used to look up the session data in the database. The session data will probably have additional information such as creation date, last request date, username, expiration data, and maybe a status (keep alive etc) and any other ID's...