You should call the police when you believe a crime is about to be committed or has been comitted. Cops would much rather hear from people BEFORE the crime than after the crime. For example, if you see a person you don't recognize as a neighbor, in your neighborhood looking into parked cars, call the cops. We would rather try to catch him and identify him before he steals the car, rather than have to chase him (endangering lives) after he steals it.
You should also call the cops about disturbing choices people make that can effect your quality of life. If someone puts their stereo on so loud it bothers you, you should go ask him to turn it down, and if he doesn't then call the cops. If there is a loud party next door and they keep waking you up, call the cops to make them turn down the noise or break up the party. Call in if you have drug dealing or prostitution in your neighborhood. You should not have to put up with that crap.
There are many ways to get your local cops to help you, that is what they are there for, you dont necessarily have to call. If it is an ongoing problem a letter or email to the chief will usually get the ball going. Or you can wave down an officer and tell them what the problem is, they are public servants that work for the citizens. A lot of serious crimes have been solved because a citizen passed along information to an officer.
DICH GIUM VOI
