It is a great way to show dedication. If you interview in the open period, even if you apply later on, you're sending a signal to the school that you're taking it seriously.
It's also extremely convenient. I live in California, but I really want to go to Duke... but it's a long plane ride. By doing the open interview, I was able to tour the campus to research my essays and also get the interview out of the way, saving myself a few hundred dollars in plane tickets/lodging.
Another reason for the open interview is that the school has an early action round... this lets schools keep this round shorter than the other rounds because it will only have to approve or reject the applications (one stage) rather than choose which ones to interview and then, afterwards, which ones to accept (two stages).