An open bar is one of the best ways to make a party about 10 times better.
It is also interesting because of the number of people that will get plastered. It's interesting to talk to family (in the case of a wedding) or your boss (in the case of a corporate get together) when either of the two of you are drunk. I think open bars are by far the best way to find out about the skeletons in family or co-workers closets.