Technology

"Polite restraint" here is a political tool. It signals to outside observers that you're not out for blood and brutality, which isn't just a moral point but also an economic calculation: If you're interested in long-term trade with a given economic area, a brutal and destructive government just doesn't instill as much confidence in stable and reliable agreements. Brutality is more likely to galvanise resistance and lead to future instability, trade disruptions etc.

A party that makes a point of demonstrating adherence to some international standards of "polite" warfare, no matter how performative those standards are, is just a more attractive trading partner. If they'll abide by those norms and (partially unwritten) agreements, that is an indicator they'll also abide by whatever contracts and agreements you make in the future.

In a way, it's circular: they show polite restraint to demonstrate their willingness to prioritise the values it signals over petty vengeance.

There is also strategic value, both in avoiding the type of galvanised resistance I alluded to above by taking it slow and in gradually building up a "new (worse) normal" for the opponent's populace, such that the relief of peace will feel like an improvement and thereby make that peace more popular.

Of course, moral considerations will also play a part, but international relations unfortunately don't often indulge in the luxury of morality for it's own sake. It just happens to be a desirable byproduct sometimes.