OK. So a hardwood floor expands when moisture level goes up. Got it. The installaton steps to install quarter round says to make sure you nail into the wall and not the floor because the nail will prevent expansion into the "expansion gap". But I just filled the entire floor with thousands of nails...won't they prevent expansion? Why am I worried about this one nail and not the rest?
Your subfloor expands and contracts too. When it does it takes the hardwood with it, nails and all. If you were to hard nail the quarter-round to the floor, and the floor contracted, it would pull away from the wall. If it were nailed to both wall and floor, it would likely split. Expansion and contraction can literally exert tons of force on a structure. That is why building techniques have evolved that compensate for those forces. Allowing the floor to float under that molding is one of those techniques.