I don't understand what you're saying - say someone has a busted headlight or is speeding. Police stop the car, which gives them a chance to talk to the person and let them know what's the problem. If it's their headlight - maybe a warning given that the driver promises to change their headlight. If they're speeding the cop can ascertain if the driver is for example sober or not. By this stop the road is indeed policed to be safer.

The being able to stop a serious criminal is just a bonus.