It updates the software that the phone used to determine which cell tower to use. It does not necessarily mean that your service will improve on every update but it if you travel its definitely wise to keep up to date. When Verizon adds new towers or adds more towers for roaming, you'll need the new PRL to access them.
Originally Posted by Dustoff-00
FYI, *228 option 2 works on all Verizon wireless phone but the PRL version will be different (at least it is on my wife's Droid 2).