Hi Johan,
The server has no connection to the browser (*) - so you do not know what the user is doing on their machine.
Hence you have no idea when they navigate away from your app, close the browser, turn off their machine and so on.
this is why there is a Session Timeout setting in the WebServer procedure. It defaults to 15 minutes of inactivity.
Incidentally logging out is not the same as deleting a session - when a user logs out the session (usually) remains (unless you've set the option to Delete Session on Logout.).
(*) you could theoretically use web sockets to know when the user has left the page, but that would be very complicated to do in a reliable way. Frankly, I don't see much point in going down this road...
Cheers
Bruce