A page layout, which stretches or shrinks to fill the browser window according to its size, is called, fluid layout. Advantage the view will adjust according to the size of the screen, ie page will adjust itself to browser used in different devices like mobile, tablet, desktop etc.
Usually, blog templates has fixed widths for most of the fields. But those who are using fluid layout may want the facebook comment box to be fluid too. Or those who does not know the width to be set can also try fluid width as it will fill the container. In blogger, filling the container will be nothing but taking the width of the side bar or body where the widget is placed.

Facebook comment box