This Sessions Article is one of the better explanations and tutorials for using and operating sessions in a rails environment. I recommend giving it a read if you're going to be using sessions are are unsure you are using them correctly.
In some apps (some may argue most) session management is a critical aspect for performance and control of the system. From a sysadmin point of view it doesnt make a lot of sense to let a session stay connected forever since it will hog resources that could be put elsewhere.
So here is a way to expire sessions that become inactive in rails: