The main idea of a notebook is portability. It enables one to have a computer under any conditions – in an office, while walking or travelling. One can put it on one's knees, on a collapsible table on board an airplane, etc. But where can we settle a mouse?
Certainly, there are compact manipulators for notebooks. But the mouse is still the most convenient device for controlling a cursor. How to provide convenient use of the mouse if there is no wide horizontal surface available? One can use one's own knee. But it is normally not very wide and not quite flat. A contradiction occurs: it is necessary to have a flat surface for convenient work of the mouse, but there is not enough space for organizing such a surface. MouseMattresss, the development of American designers, looks like a plastic box with the upper surface size equal to that of a standard mouse pad. Its bottom part is provided with a round opening. It is analogous to a stool. MouseMattress is reliably placed on one's knee and there is enough space for manipulating the mouse.

A flat pad turned into a tridimensional object (passing to another dimension), which provided a что дало resource for coordinating its shape with that of a leg.

Information source: http://www.mousemattress.com.
Images from the site: http://www.mousemattress.com.