Similar law in Norway IIRC. The idea is essentially to put you (the soldier) in the situation where being threatened with punishment for disobeying the order is weighted up against the threat of punishment for obeying an unlawful order. Basically, if you’re given an order you honestly believe is illegal, it’s a lot easier to stand by a decision to face the consequences of disobeying it if you know that following the order can lead to the same, or worse, consequences.