That's easy. Person gets kidnapped, government surveillance camera helps police find the car before the kidnapper kills them. Or, probably more common: murder happens, government surveillance camera helps police find murderer and jail them before they kill someone else.

That's why these cameras are so prevalent, the case for them is extremely obvious and easy to make (give police more tools to stop bad guys), while the case against them is a lot more subtle (human freedom, government abuse, expectations of privacy, risk of data breaches, etc).