Currently for 2022 sailings, if you pay off early your booking window can open early. They are still allowing final payment 60 days out from sailing but booking port adventures and onboard activities open before that date depending on CC level AND only if you are paid in full. Other reasons to pay in full include some other advantage like if you open a new credit card and need to charge a certain amount in one month or three months - this can be used to help you get to that reward also.