It's simply a traditional way of displaying the lovely, large gardenia blooms. Camellias are often displayed the same way. It's one of the many elegant Southern traditions.

Gardenias and Camellias don't always have the type of stem that allows for nice artful arrangements. Floating the blossoms allows the "face" of the bloom to be seen.

I have always heard that touching the gardenia blossom will make it turn brown. I don't know if that's true, but I always try to avoid doing it.

Magnolia blooms are also nice in a bowl of water.If you put ice in the water they can last a long time.