Why do criminal laws set both minimum and maximum sentence is for crimes?

Because each crime has a different caliber of seriousness. Someone who shoplifts vs. someone who robs a store are two different categories. A shoplifter will normally get a slap on the hand and restitution for stealing, as a robber can face up to 10 years in prison for holding up a gas station. Someone who is caught drinking and driving could face up to a year in jail and a significant amount of money that will be owed to the state. The same person get pulled over a again now will face a longer period of time in prison up to 5 years and up to a 10000 dollar bond. Minimum and maximum sentences are set up to separate those who are high profile criminals from those who are not a danger to society. Hope this helps:)