Some programs have their open house before officially making admissions offers. What this actually means may vary from place to place, but I can tell you that I've heard of some programs that do this just as a safety precaution. You're basically in unless you show up and are a complete ass. For example, one year one of the students pissed off all of the current grads in the program by being a prick, and of course the professors ask the grads about the prospectives afterwards so that student was not offered admission. This is more feasible in smaller programs. By the way, this was at Cornell but not the physics department. I heard from a professor in a biophysics department at another school (that also visits before officially admitting) that they reassure the students during the visit that all of them will be offered admission, so again it makes you ask what's the point. I think it's just in case someone turns out to be a complete weirdo / douchebag / creep. If you don't stink to high heaven and don't piss anyone off I wouldn't worry too much. I don't think they'd fly you out there if they didn't want you to attend. It's not like you'll be paying them to get your degree there, they pay you.
They could also do this to better gauge interest from the students in that the ones who would attend an open house are potentially more likely to attend. It would make sense if the budget situation cannot tolerate a large deviation from the anticipated number of students.