The use of shredded wood mulch, while wide spread, is fairly recent in origin, and there is no natural equivalent to this covering of shredded wood.
Why mulch? To insulate, to prevent water loss, to inhibit seed germination and the growth of unwanted weeds (by blocking light to the soil), but by far the most common purpose it simply decoration. It looks pretty. Frequently this shredded wood mulch is dyed (either red or black) but more commonly black. Use of black dyed mulch is not recommended as it can stain concrete (or carpeting if tracked indoors) and because the dye used to color the shredded wood is sometimes a preservative or chemical bidding agent and often an industrial by-product from incomplete combustion of petroleum products. These dyes are sometimes carcinogenic and actually harmful to plants and soil.
A far better mulch, and the one one finds in nature, is leaves (see the posting for November 2017)