i believe the use of water vapor is (to help us breathe), rain and clouds to block the sun for shade where there are not many trees. also, it creates fog so some people can paint it.
my main reson is the cloud one. when water vapour gathers together, it creates clouds. when it is hot, people playing tennis, footy, ect. want shade and there aren't any trees or shelters, they can rest in the shade that the cloud created.