The fathom (six feet, or two yards) is one of the many old English measures now becoming unfamiliar. When we lose a word, we lose bits of our poetry (in this case one of the most perfectly fashioned bits: Shakespeare's Full fathom five thy father lies). It was used most typically for taking soundings of the depth of water. It is the height of a human (roughly; feet and yards used to be variable). But it was defined as the span of the arms outstretched, fingertip to fingertip; it's easier to measure things by stretching out the arms than by lying down. These two bodily lengths happen to be about equal. Leonardo's celebrated diagram of various proportions of the body, frequently borrowed as a sort of hieroglyph for humanity, shows a man inscribed in a circle, the circle being touched by his head and sole and outstretched fingertips; in other words, he is inscribed in a fathom-wide circle.
So the fathom is a doubly-natural member of the set of natural measures derived from the body (foot from the foot, yard from the stride, ell from the arm, cubit from the forearm). Such measures are useful in describing that is, in the activity in which a human visualizes a scene and tries to get another human to visualize it because they save a step. If you tell someone that a wall is a foot in front of him, he immediately sees it; if you tell him the distance is thirty centimeters, you're expecting of him a mental process that is like consulting a table or using a calculator.
The fathom is neat for describing scenery and architecture. If you say The hedges are more than a fathom high, there's an immediate picture of hedges too high for standing humans to see over: a scene drastically different from one in which the hedges are lower than a fathom. I have to stoop through the doorways of Church Cottage because they are only a fathom high, built two centuries ago when people were shorter. And it is when the water becomes a fathom deep that I begin to get out of my depth. That's why I quixotically would like to revive the fathom.
As the fathom is a good human measure for describing height, so the pace is for length. A yard is a complete cycle of two (for me small) paces); two yards make a fathom.