So why were medics ever shot at?

It is well known that medics are not supposed to be shot at; this benefits both sides in theory and the wounded would tend not to go back to battle immediately if ever.

There are benefits as well:

• Medics don't shoot back
• Easier (static) and bigger (surrounded by wounded) targets
• Ambulances are dream targets: shoot one, get three or four hits

This is such a problem in the Middle East, where some people have very limited views on rules of war or the Geneva convention (when it doesn't benefit them) that the Israeli army developed 105 mm armored Merkava I ambulances.

Obviously, those tanks carrying wounded are not protected under the Geneva Convention. When asked why they did it anyway, a general replied that a 105 mm cannon and armor are more effective protection than a piece of white cloth with a red target on it.

The answer to "Why was the provision even necessary" is simple. Killing the other guy's medics makes the other guy less likely to receive timely medical aid (because the medic is either dead, or has to be much more cautious moving around the battlefield). The availability of medical assistance on the battlefield is critical to both unit maneuver and morale. Source Basically, as wounded troops pile up a unit must either handle them on their own (losing combat power as infantrymen try to tend wounded comrades) and either move the wounded as the unit moves (which makes it harder for the unit to move) or leave them (which is damaging to morale.) Even if the wounded are left untreated and the unit isn't supposed to move it's still damaging to morale for obvious reasons. Gutshot johnny screaming for his mother for hours or days as he lies untreated in the trench is psychologically way more harmful than the same soldier quickly being recovered by a medic, placed in a helo, and sent to the rear. To say nothing of the fact that prompt medical aide (referred to as "The Golden Hour" by US forces) is a strong indicator of whether a wounded soldier will survive/return to duty. Source

Killing the enemy's medics when, say, they're in a static defense (where the wounded will pile up and sap morale) or while you're in maneuver combat with a faster enemy (slow down the enemy or force them to abandon their wounded) can have an advantage even if the enemy is also doing it to you. If your army has a poor medical setup (few doctors, worse training, whatever) it also helps "level the playing field" as far as returning wounded soldiers to combat. Many wounded soldiers could return to combat in a matter of weeks or months... unless they fail to get adequate medical treatment in the first few hours/days. Then maybe a leg wound gets gangrenous and leads to amputation, or he bleeds out and dies, or whatever. If your enemy has plenty of well trained medics and you do not, it might be strategically sound to target their medics because even if everyone starts shooting medics it'll hurt them worse. This is especially true of armies that have a good system and history of caring for wounded, as the lack thereof causes more morale issues. (I don't have a source to hand on this but in WWII the US army, which had a good medical system, noted that the time it took a wounded soldier to receive medical care had serious effects on how aggressive forces were. IIRC if medical care was taking more than 5 hours morale problems were noted. Whereas less than an hour resulted in increased morale because troops believed they were being well treated. But the Red Army's troops, which historically expected no real medical care for up to several days, had a much higher threshold for medevac times before it impacted their morale.)

As to why medics were shot at besides the above, well there are any number of reasons starting with "doesn't realize it'll result in their own care being degraded if the enemy starts shooting their medics" to "hates the enemy so much ANYBODY in their uniform is a target no matter the consequences." Which is why the overall rule needed to be put in place.

In any combat situation there are some people who:

• Hate the enemy so much that they want them to suffer and die, not just lose a war
• Can't recognize a medic when they see one, or don't check before shooting
• Don't understand the logic in the question and just think the more of the enemy die the faster they will win the war
• Think that the fear engendered by shooting medics outweighs the downsides
• Don't care about morality or think it doesn't apply in war
• Enjoy killing people
• Are confused or panicing because people are shooting at them
• Think doing immoral things makes them tougher
• Are drunk or high

Any one of those will cause a person to shoot at medics, ambulances or similar things. These can apply to individuals, army units or entire armies.

And as jwenting says, sometimes you are just shooting at the enemy in general and the medics just happen to get caught in it.