It's not my culture but my understanding is that traditionally it was a formal coming out event for young ladies in the South, usually at sweet sixteen, wherein they were introduced to potential beaus. These days it's often a charity event such as this one Rachael participated in.
I see it as a chance for young women and young men to dress up and learn formal manners.
I think this one decreed white dresses and the men in black and white. Otherwise I thought Patrick could have worn his very dashing new LP costume!

But then he wouldn't be able to take off that jacket with the attached vest!