**Not no to slapdashgrim, he beat me to the post**
No, SESSIONS are used for many things. They are used to track guests, page views, you can log how long people are staying at your site, security tokens, user authentication and more.
When I work on my app that users inter act with, I use the session to store their User ID, User Name, Login Time, Their Power level (then match with DB) and a few other things if needed.
So if the user is not logged in, you can create a variable say UserLoggedIn and have it equal FALSE if they are not or TRUE if they are. This also adds a little more security.