Parole is an opportunity for a prisoner who has served part of a prison sentence to transition back into society. The parolee serves the remainder of the sentence in relative freedom, with the opportunity to lead a constructive life. The restrictions on parolees ensure that they still "do their time,” and that they keep on the straight and narrow afterward.
The possibility of parole also gives prisoners an incentive to stay out of trouble while behind bars. To earn parole, a prisoner must usually have a relatively clean behavioral record and meet basic eligibility requirements. Part of the idea is to promote a better behaved and less violent prison population.
Parole also reduces prison overcrowding, while granting offenders who are unlikely to harm others the benefit of supervised life in society. Parole helps the government try to cut down on the high costs of maintaining large prison populations while keeping the non-prison population safe.