We haven't taken ours ashore and were never saked for them, but I have read in the past that some ports (sorry don't remember which one) require them, I am sure if they were needed the info will be givne to you on the ship. Now, IF for some reason you miss the ship after a day in port you will probably need to fly to the next port (at your own expense) to catch up. I'm pretty sure you would need the pass port for that. IF there is is a reason you have to cut the cruise short and go home for any reason you will have to have the PP.
If I am wrong on this, I'm sure someone will correct the error.