All the cruise lines have a "loyalty" program. The "rewards" they offer to repeaters vary from line to line.
After 1 cruise with a line, the next time you cruise with them you'll be invited to a repeater's free cocktail party.
Since you've cruised HAL any Carnival Corporation ship you cruise, you'll be considered a repeater, and invited to the cocktail party.
You'll also receive some "repeater" offers directly from the cruise lines, but I find you can generally find better pricing on your own.
After a lot of cruises on some lines the "rewards" get somewhat more substantial, with priorty embarkation and debarkation, concierge service, free laundry service, complimentary internet time, etc.. Again, it all varies by cruise line.
Royal Caribbean offers discounts. Once you join their repeatersclub you get a magazine. I have saved money on a cruise i booked by using one of their coupons from the magazine. I had alrready booked the cruise then found the coupon and it was fror $100.00 OnBoard Credit.
I just called my agent and hey applied my coupon to my booking.
Yes there are some money saving benefits. Royal Caribben offers some good perks after you become a diamond member.