"The aim of the Gothic architecture was to achieve light looking, vertical buildings. So they had to invent ways to handle vault pressure without heavy walls. With flying buttress it is possible to keep inner walls thin because: the flying buttress' design provides for an equal and opposite force to be imposed on the wall, thus keeping the wall in balance. This, firstly, enables the vaulted roof and, secondly, by externalising some of the structural elements of the wall, allows the wall so supported to be thinner, which in turn enables the development of large arched window sections to let in light and be filled by stained glass (source: wikipedia)."
