Attacking some civilian targets does have a strategic advantage. First, attacking factories can deny resources. Second, making a population tired and stressed can lead to issues at home that need to be taken care of, which takes manpower and resources. I’m not condoning it, but it does create some strategic value. That’s what the bombings of cities were for during WWII. It was largely about destroying war infrastructure (with hard to aim weapons and poor compared to modern intelligence).

War prisoners also take resources to care for. If they’re dead, they don’t. It’s potentially advantageous to not have them. Again, not condoning it, just stating reality.

The Geneva convention covers many things. It’s a set of guidelines to ensure war doesn’t escalate. There’s some things that are banned just so it’s not confused as another form of attack and things spiral. It only works if both sides of a war agree on the rules though, otherwise why is one side not allowed to use tools their enemy is using?