Bonus Bucks have been around for a long time. (If you do a search for the term on this forum, you'll get a lot of hits).
Sometimes they're given as an enticement to stay at particular properties at particular rates.
Sometimes they're given as enticements to buy products from "partner" companies (I believe there was a deal not long ago with HP printers or some such that gave multiple $100 Bonus Buck certificates with the purchase).
Sometimes they're given in relation to other Marriott products (a couple years ago you could get a $25 Bonus Buck certificate free when you bought a Marriott Gift Card for $50 or more).