With Medieval cathedrals, building high caused problems. The weight of the pitched roof didn't just push down on the walls. It also had a horizontal component and pushed sideways at the top of the wall. This pushed the walls sideways and they collapsed. One way to stop this was to shore them up using wedges of stone called buttresses. But these were regarded as ugly so hence the more elegant flying buttress. This is essentially a heavy column with a lot of mass. Then there is a half arch to the top of the wall. The idea is to turn the horizontal force into a vertical force in the way that an arch does. There are only a couple on Tewkesbury Abbey so these might be the remedial needed-to-fix-a-problem type.
