Expect for the concept of free-rooms-bulk that simply must be baked in it's really about this issue
Providing support for better negotiations in the future
If you promise year after year and fail in the end they do not believe you anymore. For most of the hotels this is also difficult to track. It is not different to corporate deals. If you book with your corporate price, they can count your stay in reports because most of the time only indicator in their PMS system about corporate affiliation is the price type. If you find better/cheaper fare in the very same hotel, the hotel is unable to track you as that corporates traveller, eventhough you made your trip for business. Same here, if you book outside the arrangement and do not mention the group/agreement code, for the hotel it is equal that you did not attend that group event. Seen too many report mismatches between corporate recorded stays in specific hotels compared to what hotels are telling.
CP