Barleywine
For what purpose do you use tarot cards?
If I were a really pretentious mystic poet, I might say: "To probe the interstices in the fabric of the Universe."
My poetry is bad in an entirely different direction, so I'll just say:
To gain insight and wisdom that elude conventional reasoning and transcend the prosaic view of reality. These yields might be put to different uses (prediction, empowerment, etc.) but the goal is the same: to get "under the skin" of appearances and overt behavior in a way that sidesteps conscious preconceptions and biases, granting a unique, impartial perspective on the matter at hand.
Oh, and it's fun as well as (often) metaphysically profound.