Why Are Postmortems Carried Out?

Postmortems are carried out to establish a  cause of death.  Usually this is done by the coroner, who may wish to ascertain the exact cause of death, or to assess the process of a disease or to assess the treatment which had been carried out immediately before death.
So if a person who had cancer but was expected to live at least a couple of years, died very suddenly, the coroner may wish to have a postmortem to ascertain whether the disease had in fact spread very rapidly or whether the cause of death was something else.  If a family are suspicious of the medical treatment which had been administered to the person who died , then the postmortem could show whether this treatment had been correct.
Relatives can appeal against a postmortem being carried out, since it can be distressing for people who have suffered a bereavement, but most families agree, since they realise that it can help find out exactly what caused the death of their loved one.