Reason behind small hole in the window pane

On the way of ascending the altitude of the airplane, there is a constant decrease in the air pressure outside the plane and increase of air pressure due to the bleeding air conditioning system in the plane. Following the difference inside and outside the plane, the air provides stress to the cabin and especially the windows.So,  concerning that way, there are three layers of glass in the window. They are an inner, middle and outer layer.

The hole has been created on the inner plane to prevent the window from fogging and freezing which may happen as there are temperature and pressure variations. In case the outer layer shrinks then middle layer acts as a barrier. The hole balances out the bleed air protecting the outer plane from physical stress.