Portal session expiration

Although our Portal log shows that user sessions always expire approximately 60 minutes following login

, regardless of the visitor's most recent session activity, visitors are not informed that their session has expired and are able to continuing navigating to various portal pages, creating and updating records for several hours following their expired session.  

Can someone help me understand the purpose/significance of sessions given that session expiration appears to have no functional effect on the ability of authenticated visitors to keep performing their portal business?

Thanks,

John