Many time this is done by the Travel Agent as a thank you for using them. I have a $200 credit for booking through Princess direct on a special sale. There are many reasons, but it is always nice to spend.
MAny times, if the cruise line lowers the cost of the cruise (after you've paid) they will give you a shipboard credit. This helps ward off the event that you might find the price lower and want it - by giving you the credit before hand, they've addressed the issue and all parties are generally happy.
We also have $75 per person for our February cruise. It is not a thank you from our TA, I asked. It is not a future booking credit as we did not go that route. Obviously it is some sort of promotion, but which one, I have no clue. Was just happy to get it!
That's funny- because when I called Princess directly for prices on cabins for our Feb. 18th cruise on the Dawn, they wouldn't match the $100/pp shipboard credit that Expedia was offering us for the exact same cabin, same exact price, except we were getting the credit. I even said, "well I guess I should just book with them, huh?" and they said yes. Go figure!
Laurie - It must be a travel-agent only bonus then. Booking directly with the cruise lines is a different thing sometimes. Hope you got a good deal with Expedia. To make sure, check a few other websites.