Yes, that is correct. As sites get larger, it becomes harder to host a site off of one machine.
Many sites use commercial grade load-balancers to transparently switch users between machines.
In order for sessions to really be persistant, they have to be accessible by all servers in the 'farm'. Using a database to store a restorable version of the session is a widely used solution, and these functions allow for just that. It seems more transparent to the code writters and less prone to errors. All rights reserved.