Postmortem drawings are done from unidentified fleshed remains. The medical examiner or coroner has not been able to get an identification through standard means, and an artist is called in to draw the victim. This is done when there are postmortem changes to the remains (decomposition, animal activity) that would preclude releasing a photograph.
The artist draws the victim without these changes, and the drawing is released to the media in an attempt to get an identification so that the family can find out what happened to their loved one. If this is a homicide case, the detective will be able to progress further with the investigation once the victim is identified.