At the National Center for Atmospheric Research in Boulder CO is an exhibit where a person is able to pump up the pressure in an enclosed space, and the release the pressure quickly. The result? Fog forms. So probably the pressurizing and depressurizing of the plane could result in fog (and thus moisture) forming in a sealed environment such as between two windows. Thus the hole helps to keep the pressure balanced (Yes, I stayed at a Holiday Inn Express last night. )