What Should The Police Be For

1.  How often, under normal conditions, does police presence prevent crime?

In the broad sense, this is difficult to tell because prevention is only measurable in a relative sense, in comparison to the lack of police presence under the same conditions.

For mobile crimes such as street drug dealing, the crime can move to a location where the police are absent.  This is not so much crime "prevention" as crime "suppression".

2.  What about law enforcement.

There are many law enforcement agencies in addition to the police:  Food inspectors, building inspectors, bank inspectors, etc.  What differentiates these agencies from the police is that the police are empowered to use physical force to compel following their orders.