Woolly Slippers on the Fruit Trees

Why do our fruit trees have wool around the base?  Using sheep fleece is a traditional way of mulching fruit trees.  We place the fleece around the base of the fruit trees and it acts as a mulch – i.e. it suppresses weeds and retains moisture in the soil.  It also slowly breaks down, which releases a steady source of nutrition for the trees.  Lastly it is appreciated by the birds for use as a nesting material.