The reason builders work in that range of moisture content is because the wood is most stable in that range. If it drys out too much the wood shrinks, and if the moisture content gets too great it swells. Going to the extremes is where wood warps and cracks. Warping occurs because it swells or shrinks in a non-uniform manner. That is main reason builders always want to use quarter-sawn wood, because it helps to minimize the non-uniform changes that occur from shrinking and swelling.