Think about it like this...I don't know if you've ever had a worm bed, but the worms will feed on top where the OM has been added to the top. Not only do the earth worms feed off of this OM so do all the other microscopic critters in the soil that has been talked about above. With the OM being concentrated so will the digested waste being deposited by all the critters feeding on it, this is what is so nutrient rich and in an available form for the plants to utilize. Another way to look at it is with composting. If you take all the materials in a compost pile before they are composted and just scatter them and cut them in, they will not breed the same beneficial fungi and bacteria that they will if they are in a left piled together. It's a little different in that these layers aren't think enough to go through a heat like a compost pile but there is a decaying layer there that creates a better environment for the critters to flourish.