A Prison Place costs £38,000 per year, Probation Staff are paid a tiny amount in comparison and one probation officer can manage upto 50 such individuals, an absolutely massive saving.
In addition whilst in Prison the offender is not contributing to society in terms of tax and insurance, but if you get them into work then they become a productive member of society again.
It is important to remember that ALL type of people commit sentences which may mean less than 12 months in Prison (take drink driving for example) it is not just drug users and hardend criminals, so the chances of finding work are increased.
Also sentences of less than 12 months are often for less minor offences but essentially the offender could lose their home, income & family. Dealing with such sentences in the Community actually helps society in the long term by saving money, which means less tax rises.