You get a shipboard credit for that next cruise and your agent can also get credit for the booking. The pricing is not supposed to be any different than what you could get by booking it from home.
Dave is right. If you book a balcony cabin you would get a $100 on board credit to use while on the future cruise you booked. That's about the only savings you get. I do it all the time. If I change my mind like I did this week, I still get the credit as long as I keep the same booking number.