I've actually had my boat in one of those...
A mix of large power and sailboats. I forget what it's called. When you have enough boats to complete a full circle, everyone can go from boat to boat via swim platforms (really helps to have one!) If the circle is big enough, it provides a protected area for swimming that lets parents keep an eye on their kids...even a relatively small circle makes for fun potlucks and "everyone bring a heavy h'ordoevre" cocktail parties.
It can be a be challenging to tie up, 'cuz sterns have to be lined up perfectly and bows apart, held off by tension in the lines all the way around the circle. So fenders on the stern quarters are usually all that are needed.