What is the purpose of the set sail pass? Can't Royal just type in your reservation number at the port and pull up the required info? What stops people from changing passenger names or C&A status on the pass? Seems like a security concern to me.
The purpose is to get passengers to fill out all required forms and documentations before they arrive at the pier. This in theory speeds up the check-in process for everyone. A passenger with a set-sail pass will only have to show their passport and swipe their credit card at the check-in desk before receiving their room keys.
A passenger who didn't print their set-sail pass will have to wait in line to fill out all forms and supply Royal Caribbean all passport information at the pier which takes considerably more time and slows down the line behind you.
It's the same theory as printing out an airline boarding pass at home before you go to the airport.
As for preventing fraudulent documentation... that's what the barcode, reservation system, and your passport is for.