wowzz - I think it's partly to check that you actually have somewhere to stay and partly so they can contact you if needs be. They rarely if ever check it out, but I guess they could if they wanted.
I've had interesting conversations with immigration, firstly when transferring flights at LAX on a AKL-LHR trip when I was only in the US for 4 hours and secondly at IAD when my first night was on an Amtrak sleeper service. Slightly raised eyebrows on their part, but it was all treated fairly humo(u)rously.