A PI's work is so vital because police agencies are severely understaffed with the increase in violent crimes. Police agencies have focused on more serious violent crimes. That has resulted in a severe gap between the resources needed to solve crime and complex community problems. In the Western world, there are several signs that the Police agencies devote fewer resources to solving fraud, property crimes, and missing people.  

That's where private detectives come in.

There are many fields where they can act efficiently, such as corporate investigations, civil investigations, infidelity investigations, missing heirs, missing persons, infidelity investigations, workmen’s compensation investigations, insurance fraud investigations, accident reconstruction, etc.

More at:
https://www.blackenterprise.com/maurice-hicks-private-investigator/