Just that the measurement is independent of enough details so that it's relevant no matter which bicycle you're looking at. As you'll agree, a straight ratio such as 48/16 is meaningless because that's a fairly tall gearing on a 29er, but not so tall on a folding bike. Gear inches takes the wheel diameter into account so that 56" on a high wheeler is comparable to 56" on a safety bicycle. I applaud Sheldon's efforts to factor crank length into the "effort" of bicycling, but without those details (the OP never posted them and hasn't returned), it just muddies the discussion. And like I posted before, if the OP had 170mm cranks on both his mountain bike and fixed-gear, it would be a moot point. Just to move between wheel sizes (as in your first post), all you need is gear inches. (Or meters-development, if you prefer metric...)