Why do Tree Services Leave the Grindings?

Why do tree services leave a mound of debris after they grind your stump?  I asked our tree service, Advanced Tree Care, that question and what they told me is this:  The mound of grindings will naturally heat up, just like a compost pile (which it essentially is) and that heat will help kill out the stump so it won’t grow back.  Eventually the mound will collapse and you can remove the left-overs but you want to leave it there for a while.