It shows investigators that the shooter placed the gun directly on the skull of the deceased. This can be used as evidence to counter a suspect's claims to the contrary.
Few criminals understand the power of forensic science.
This man was hit repeatedly by a hand hatchet. The slug was found by accident during the autopsy. Such evidence can be concealed by other injury or decomposition. The quality of autopsies can vary widely and no standards exist for them. The vast majority of deaths are rather predicatable as to the cause and conclusions easy to come by to the point even a novice can come to the same conclusion as an ME, however the better Medical Examiners can harvest huge amounts of information from what appears to be an obvious cause of death.