The half-circle sofa can add shape to a boring rectangle of a room, it can complement the natural curve of a room, or it can do a good job of demarcating space in a large, open floor plan. It's good for conversation, too.
The sofa was a custom design from the Interior Designer, who then had it made in New York with a vendor that they use. You could take the idea to any custom furniture maker to do something similar. Thanks for the question!