Provide a simple way to identify session among a group of HTTP/HTML requests. The cookie value is often an index into a table stored in the memory of a Web server that points to an in memory object holding the user's records. This has many potential problems: If the user's request is routed to a different server in a subsequent request, the session information is unknown to the server.
If the user is routed to a different server and the server is part of an application cluster, then all the servers that could receive the user's request must have a way to synchronize the session data. Storing cookies and synchronizing sessions among clusters of server usually requires configuration, storage space, and memory.   _{Your Comment}