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Builders believed that vaulted ceilings helped to prevent fires from spreading which was important because there were no formal firefighters in Gothic times. They were difficult to build because the vaults were so heavy, so not only were they hard to get up there, they created engineering problems because the extra weight caused more lateral force to be exerted on the load bearing walls. To compensate for this pillars or columns and buttresses were used. The buttresses reinforced the walls and columns were used to help support the weight.
Once built vaults are very strong, like an arched structure. They can then help support the rest of the structure, very complex engineering/physics involved so can I suggest your own reading because short answers - even good ones - will not give you a good understanding of the matter, just a pass mark. Knowledge is good for its own sake and when you get old like me you will be glad of it.
@ Cryptic: Glad I'm not the only coot on this site. lol