as I understand it, in nutshell, updates new towers/trans sites.
For example old tower 2 miles away, new tower 1 mile away, without prl update phone will search for and use the old tower as primary site. That can cost you battery life. PRL update should give you best/shortest/strongest route for your traffic. So in theory up to date PRL should give you best signal and battery life.
If you want more tech details just search the forums. In the past I have read several from knowledgeable people in the forums.