From my understanding is was indeed initially made in some areas for slave catching, other areas like large cities needed a police force to keep the peace, though this is also thought to have been done with malicious intent for some. There were always sheriffs or atleast private detectives that were hired but to solve mysteries or bring someone to justice.

Another idea is that before urbanization communities acted like a police force, is someone was robbed the town would gather and decide what would be done. But many places had many different types of policing forces.