We can argue that convicted criminals are better able to handle society when released on parole because they are sort of eased back into that society instead of being thrown back in with no support.
Convicts who are simply released back into society at the end of their terms typically have little to nothing in the way of formal support. If they are lucky, they have family who can support them emotionally and perhaps monetarily as they try to reenter society. However, many convicts lack such support. This can make it very difficult to move back into society, particularly for convicts who have served long terms.
A parolee does not necessarily have any emotional or financial support, but they are at least given a transition period. If they are given support (as in this link) it can help them to find work and to regain the ability to function in society after years of being in a very closely supervised situation. Even if they are not given such support, they at least have a transition period. In this period, they are under supervision and have greater incentives to refrain from criminal activity. This, at the very least, helps them to get used to society again before they are forced to rely solely on themselves to keep “clean.”