Use your imagination. Four billion years of falls of everything, twice a day oceans, iceblocks, rocks, the volcanity activity that takes place inside the earth and plates moving. If they didn't also contribute to spin of the earth it might have slowed it down. A rock falls, it either contributes to the spin or gets dragged along. If it gets dragged along it slows the rotation. To contribute fully it must have a momentum of it's own. All objects on earth would orbit the centre of the earth if the path was clear to do so, it seems to me that they must therefore contribute to the spin.