Nobody knows exactly why, but this is a common and normal activity for dogs, especially puppies.  Most dogs do not suffer any repercussions from this, but coprophagia could lead to re-infection from pre-existing internal parasites or to gastrointestinal upset from bacterial or viral load.  Products that reportedly change the flavor (and tastiness) of the feces exist, but do not work for every dog.  If your dog likes to sample the feces, the best thing to do is remove it before they can eat it!