2. Probation is commonly used for first offenders of a crime if the person does not pose a serious danger to society. The reason for this is primarily economic. It is very expensive to keep a person in prison. There are thousands of costs per prisoner per month of incarceration. Prisons are chronically overcrowded because taxpayers resist having to pay for more of them. Further, when a person is incarcerated, not only does it cost money to maintain the prisoner, that person is no longer a working productive member of society.
Probation is not really meant as a punishment. Rather, it is a system to make sure the convicted person is turning his/her life around, staying out of trouble and working toward becoming a productive law abiding member of society. It has a punitive affect by forcing the person to meet regularly with a probation officer and to refrain from certain activities the person might otherwise enjoy.
But the main effect, if it works, is to convince the people on probation that the State will not tolerate the criminal behavior that occurred, and that if they do not show signs of correcting themselves, they may find themselves in jail next time.
It is hard to say exactly how effective probation is since many people who never commit a crime again might not have done so anyway. Many people who commit another crime might have done so anyway had they received some other punishment. But it is successful in convincing at least some people to stay away from future crimes and also in relieving the pressure on the prison system.