I often book outside the conference as the negotiated rates are often higher -- especially larger multi-hotel conferences. There used to be a time that the rates were much better, but not anymore. For example, I was at a meeting last week and the negotiated rate was $40/night higher than the cheapest rate on the website. For another meeting next month -- I got a rate 35% lower than the conference rate!

As for ID, I have never been asked when booking using the conference rate.

Some conferences also bundle in services they provide (similar to getting free space as Sopwith has said) and pass on the cost to the participants via room rate.