Rather than me type out all of the reasons why autopsies are performed here in Pennsylvania, you may click on this website and see for yourself.
http://www.pacoroners.org/Laws.php
I remember attending my first autopsy as a young Investigator. Like so many others before me, I had to be excused, so I could use the bathroom, but not for the usual uses. After my third autopsy, I became pretty much benign to them. It helps to view certain parts of the autopsy as an Investigator, so that when you testify in court, you can better explain just what it is that you are trying to convey to the jury. The really tough ones to still view are those of young children. It's an event that you just can't begin to imagine. It stays with you for a very long time.
I had a case 4 or 5 years ago of a young boy who was then 11 years old. He and his buddy were playing with one of the father's guns, which was a .357 magnum. The gun went off and hit the boy in the chest and blew a pretty decent size hole in him. We were not going to perform an autopsy because the case spoke for itself, but the insurance company required it and paid for it. They wanted to know if the child had any other diseases, like cancer, etc., so they could lessen the amount of the payout. Unbelievable.
Then, we also had an autopsy of an older man that died of what we considered to be just old age. I believe he was in his mid 80's. Again, the insurance company demanded the autopsy and they also paid for it. When the toxicology report came back, it was found that he had a large amount of ethylene glycol, or antifreeze in his system. After an extensive investigation, it was found that the two sons did not want to wait any longer to get their inheritance, so they took matters into their own hands. The insurance policy was for $500,000.00, but the old man was worth another $3mil besides. The irony was that during the autopsy, it was discovered that he had cancer, both prostate and lung and was probably in his last one-two years of life.