Why is it important to build up organic matter in the soil

What will this do to the fertility of the soil? With what results?

First it helps loosen the soil. This is important if there is a lot of clay that packs hard. It allows water and nutrients to enter the soil.
As the organic matter breaks down it puts nutrients back into the soil that plants need to grow.
example- one of the best things you can do for your lawn is use a mulching mower. It cuts up the grass into very small pieces that break down quicker. This puts nutrients back into the soil that would be lost if you bag the cuttings and dispose of them.