Why do we buy compost?

Molehill soil is quite fine, like sand in this garden. Unfortunately, the dogs like digging into the molehills playing Whack a mole, so they usually flatten or disperse the soil before I can get to it! Spud is particularly good at this. He stands, motionless, beside the growing mound, head on one side, listening. Suddenly, he'll pounce, shoving his muzzle into the mound and, often, pulling his head back out with a wriggling gentleman in black velvet in his jaws. I hate it when he does that, actually, much as I dislike the molehills in the grass, I don't like seeing them killed.