Answer:
DNA is unique to every person. No person has the same DNA. So this would be really helpful for cops because it saves them time and energy because they can do a DNA analysis and see who the colpret is/was. I think the city police should make their own database because if the criminal gets out of jail and commits another crime and leaves DNA at the scene, it will be much faster to track them if they are already in the system. Instead of doing the whole DNA analysis thing again.
hope this helped