This function was added in PHP 4.0. 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. If your sessions are complicated, with objects and arrays and whatnot, this will probably not be adequate for debugging. I've written some code to print out session info in a more readable manner.