It allows you to change- at any time- the side of the sectional that is the chaise. I love the versatility and the ability of move your room around. It would be great for apartment dwellers who move a lot.
I suspect you could even add the long cushion to the sofa and move one of the small cushions to the movable piece and have a sofa and ottoman... but I decided not to make the salesman indulge that curiosity!
In this image the piece that makes the chaise is pulled out to illustrate how it works.