What goes into a police report and why do police write reports?

In fact, I believe the vast majority of good police work is  supported entirely by an officer's ability to present accurate, detailed and informative information in the form of the police report.  Nearly every service we perform as police officers calls for the initiation of some type of permanent record.  This record may be anything from a single line entry concerning an unlocated problem to a lengthy detailed investigative document describing unimaginable pain and suffering.  

Explanation:

Internet answer from a law place.

Police reports assist with the identification, apprehension and prosecution of criminals by serving as a source document for filing criminal complaints, by providing a record of all investigations, and providing a basis for additional follow up investigations.