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Or rather, a simile:

Tacit knowledge is like the ground we stand on.

I was trying to imagine what the world would be like if there was no such thing as tacit knowledge, as we defined it-- it would be like trying to live without anything to put our feet on. Picture us getting around by flapping our (imaginary) wings and trying not to fall... we would have to do all the work the floor does for us and we take for granted. It keeps us from sinking to the center of the Earth, it provides traction, it stabilizes us, and if we fall, it's there to catch us. The ground frees up energy* that we would otherwise use just to stabilize ourselves so that we can move.

Similarly, tacit knowledge, our unconcious processing of the world around us, the things we know without knowing we know them, frees up room in our brains we would otherwise be using to conciously analyze every aspect of our environment. It keeps us from sinking to the pure stimulus-response of our reptilian brains, it provides traction for higher thought, it stabilizes us, and if we fall, it's there to catch us.