If a thicker object is placed on the glass, an ordinary hinge would leave the cover at an angle to the glass; a floating hinge raises the hinged edge of the cover to the level of the book so that the cover remains parallel to the glass, but raised above it.
If a denser object is placed on the glass, an ordinary hinge would cause the cover to tilt relative to the glass; a floating hinge lifts the hinged edge of the cover to match the height of the book so that the cover remains parallel to the glass but elevated above it.