If you get one, you're basically guaranteed acceptance. I don't think they send very many out at all, though. So if you don't get one, you're basically in the same pool as everyone else, and there's no need to worry even in the slightest. I imagine they only send them to the extremely qualified applicants who will probably get in everywhere they apply (Harvard, et al.), so that Chicago can start tempting them early. Those students and the athletes they really want.