I think it can boost you chances in a couple of different ways.
First, it demonstrates your interest to the school and potentially gets your name in front of people who make admissions decisions. Schools are concerned with their yield (if you will attend if admitted) and taking the time to attend and event makes you appear to be a safer bet.
Second, you can learn a lot of things about a school and what they are looking for by attending an admission event. I visited HBS, GSB, Kellogg, Booth, Haas, and Wharton as well as attending admission events for UCLA and Carnegie Mellon. Every school had different ways that they positioned them selves and different qualities that they emphasized. You can glean a lot of information that may or may not be available online from these events and potentially develop some additional essay material for the "why X school" question.
If it isn't to big of a financial or time commitment I think it is well worth the effort to go. If you are going to spend the next two years and $100K on a school you want to make sure it is the right one for you.