Acetoacetyl CoA is the precursor of HMG-CoA in the the mevalonate acid pathway of cholesterol biosynthesis, and is essential for cholesterol synthesis.
It also takes a similar role in the ketone bodies synthesis pathway of the liver. In the ketone bodies digestion pathway (in the tissue) it is no longer associated with having HMG-CoA as a product or as a reactant.
It is created from acetyl-CoA by thiolase, and it is acted upon by HMG-CoA synthase to form HMG-CoA. (During the metabolism of leucine, this last reaction is reversed.)