An interesting example of an oddity involving parole (or, rather, not involving it) from the American War of Independence was the Convention Army. The British army at Saratoga was placed in an unfortunate situation - perhaps not an unwinnable one, but an awkward one - and they negotiated a surrender which did not involve giving their parole. Instead, under the terms of the surrender, they would be shipped back to Europe, and would not be prevented from taking up arms once more.
The relatively generous terms involved show the extent to which the British army still "had some fight" in it - if they'd been in a worse situation then the American commanders could have insisted on terms of surrender which included the British force giving their parole.
What actually happened, however, was that once the British force had already stacked arms the Continental Congress revoked the terms of surrender - specifically, revoking the bit about shipping them back to Europe. So they remained prisoners until the end of the war.
Parole exists for a number of reasons, among them including the convenience of the victor, but also the ability of the vanquished to continue resisting. It would of course be a much more final way of not having to fight an enemy again to just kill the lot, or to keep them prisoner (inevitably under poor conditions), but the reason why "kill the lot" so rarely happens in history is that the worse surrender is then the more willing people are to keep fighting - and if "surrender" isn't an option, then they'll fight to (or close to) the bitter end.