Well, to be precise, ozone is used to treat water. However, ozone is not a molecule that's stable enough to be shipped by itself, so industrial uses of ozone re

Well, to be precise, ozone is used to treat water. However, ozone is not a molecule that's stable enough to be shipped by itself, so industrial uses of ozone requires synthesis on the spot. And the way you synthesize ozone is by zapping a pure oxygen atmosphere with electrical arcs. You can't use regular air, because the nitrogen in air tends to form nitrogen oxides instead, so you need the pure form of oxygen--which in industrial quantities is shipped as liquid oxygen.