It is intended to allow some rehabilitation in the community, while keeping release dates consistent and prison numbers down. Those guilty of more serious crimes - such as serious sexual assaults or grievous bodily harm - will spend a greater part of their sentence in jail. Therefore I believe it is not fair for them to serve the whole time while others doing the same time and crime got/get to leave on parole. But it also depends if the criminal that left on parole was more behaved then the one serving the full sentence.