My experience is that they need to know depending on what you are booking -- for example, for ressies at a few locations (Lunch at the Castle & HDDR for example) they charge your credit card when you make the ADR -- regardless of whether or not you're on DDP. But, then if you're on the DDP, you can opt to have the charge reversed if you want to use the DDP credits for those meals. If you're on DDP, they would need to explain this to you.
There may be other reasons, but that's the first one that popped in my head!