Mostly drugs and drug law penalties. If we released all people in prison for drug possession, the prison population would AT LEAST be cut in half, probably closer to a quarter when you factor in drug related crimes that aren't just possession.
The same way people refer to mild drugs like marijuana as a "Gateway drug", it's used for policework too. They catch people on a drug charge and use them to gain further information about other crimes. Or they execute a search warrant for drug related crimes and find illegal weapons, or evidence of other more serious crimes. Basically it would be like stopping someone for a busted tail light and finding a bloody corpse in their back seat. The tail light prompts the stop and then you find the murder victim. Drugs are the tail light.