I used to work at Sprint so this info may not be accurate for verizon but here it is anyway.
The PRL list is a list of non-sprint (or in our case non-verizon) towers that serve to provide service to your phone when you are not on the verizon network. It does not update verizon towers, those are updated automatically. The only affect is that you may get a better signal when roaming. For instance, lets say your house does not get verizon coverage and you're actually using another company's tower and it is far away giving you a weak signal. If that company builds a closer tower your phone won't know about it unless you do a PRL update. But if Verizon builds a closer tower, your phone will automatically get a signal from it without needing to update the PRL.