Our police forces respond and react to reported crimes or situations. They do not prevent or intercede on anyone's behalf but that of their own or of the state.
“You want to find a Dunkin Donuts, you call a cop.” - Leonard Smalls (Raising Arizona)
You really cannot rely on the police to prevent crime unless they are physically present at your location. Otherwise, they are only there to pick up the pieces, which is one of the principle arguments for the 2nd amendment.
That being said, if you use a firearm to defend yourself, you had better call the police, otherwise the question will arise as to your guilt. As far as talking to the police, it’s always better to say little or nothing until you get legal counsel and are able to choose your words wisely.
As far as calling the police if you are a witness and uninvolved in a crime, that’s your call. In some cases, you might be subject to reprisals from the criminal. In other cases, you could unwittingly become a suspect if the officer is intent on finding a suspect.
As we have seen with respect to Flynn and the FBI, unscrupulous police departments can engage in entrapment. This isn’t Mayberry RFD.