First thought is as some root protection for the tree, ie water will not wash away the supporting soil. In the main, tree roots are a mirror of the branch layout, the degree of 'macrocarpa' in essence is the tree's natural response resistance to overturning. It is all a tug of forces, too much resistance and the trunk fails, too little and the tap root is strained to breaking point. You can guage this when looking at the ball of failure on wind disrupted or collapsed trees. Some trees have a full macrocarpa of fine roots, others have a mass of large roots.
