From Bernard Chasan, a physicist at Boston University (many thanks to him for e-mailing this, and allowing me to quote it):
A friend, hearing me deplore the dearth of good classical radio stations and the increasing age of the audience, asked: why is classical music so important? My answer: there is nothing else in my experience which so allows the expression of the deepest emotions within a framework of almost mathematical logic. The combination is a very powerful one.
This was my answer ten years ago — a bit formal and abstract, but after all I am a physicist. Ten years later, approaching 70, I would add that in a way which I do not understand at all, classical music can express spirituality.