Thanks so much for the question. I think that visiting can help with two things; one is to let the school know that you have enough of an interest to come and find out more. The other is to allow you to really get to know the place so that your application is infused with a great sense of belonging- which after all, is part of the game ! In my years in admissions, I can tell you that we never held it against an applicant if they could not visit; but we also thought that if they lived in bordering states ( or on the Eastern Seaboard for that matter) it was a great idea to show up.
One thing I would suggest is to do your homework beforehand so that you know just what you may want to learn more about, and whom you may want to interact with. You want to make it clear in any interaction with students on campus, admissions professionals, or professors- that your interest is clearly well thought out, and that you are seeking relevant information about certain topics.
You may also find that on a visit day such as the one you mention at Darden, you are introduced to people and resources who can be really helpful during your application process- so you never know what kinds of extra relationships you may be able to build during your visit.
Imagine if you were showing someone around your workplace and they had done NO homework about what you do, or how your company works- as a result they asked really boring, or simple questions. You may get a little frustrated and feel like they did not really have a true interest in what you did every day. Same principle holds with a school visit.
And one more thing: TURN OFF YOUR BLACKBERRY OR SMARTPHONE during the admissions presentation!