Anyone notice that a 210' x 210' square covers just about a square acre? Never mind that acres really arn't square. Being an odd number also gives the map a center square, for what its worth.
Taken with my 880ft tactical hex, you can clearly fit 16 of those 21x21 square maps in the hex. Thus you start to have a means to which you can picture what the dungeon you are mapping is actually under!
When I saw the graphic it made me think of Moria which led to the thought about how the information of a square acre gridded out might be useful to dungeon masters and mithral grubbing dwarves who delved too deep. This also might help translating out to a battlefield map.