The advantages vary based on the cruise line and the offers they have active at the time. The benefits include reduced deposits, onboard credits, free upgrades, and guaranteed dining preferences. Your TA will get credit for the booking if you request it. We have booked a couple of future cruises while onboard and gotten very good benefits from it.
We didn't actually book another cruise while on our last cruise, we bought a certificate of $100.00 each, which will go towards our deposit for the next cruise. We also got the OBC and we were told we would be able to extend this same offer to (2) other couples since we bought it while on board.