You answered your own question. To prove the cause of death.
Autopsy after death is a way to precisely determine a cause of death.
Autopsy.
Before declaring a cause of death, the officers waited for the autopsy results.
To determine cause of death if unknown.
A postmortem examination to discover the cause of death or the extent of disease: "an autopsy report".(Perform a postmortem examination on a body or organ).
During an autopsy a body is usually dissected and it's tissue are tested. The reason for the autopsy is usually to determine the cause of an individual's death.
An autopsy is performed on a dead animal or person to investigate the cause of death or other information.
The crime victim was given an autopsy to reveal the possible cause of death. The autopsy revealed that the boy had died from lack of oxygen by strangulation.
Autopsy.
A forensic autopsy seeks information about the cause of death, the manner of death, and any contributing factors or underlying conditions that may have played a role in the death.
A kidney autopsy may be needed to ascertain the cause of death. If blood and other DNA tests come back as inconclusive, it may be necessary to perform an autopsy.
An autopsy is usually performed where a cause of death is uncertain or unknown (poisonings, for example), or to find out the exact mechanism of death (as in stabbings and gunshot wounds).