It is bad to keep things from the police.
Will witnessed a crime in his neighborhood near his home and the police are questioning him about it.
Will wants to keep his name out of the report for his safety.
Will exercises his right not to talk to the police and later submits an anonymous tip.
Will states to the officer that he did not see anything.
The police arrest the criminal based on the information and the neighborhood is safer.
The police cannot locate the criminal who is free to commit more crimes.