Question about magic damage

My build have 16 str, 35 dex, and 50 faith, so i was thinking that using a divine/occult weapon would be stronger with this toon so i made a iato +10 divine (i used an online calc and the AR was higher than the iato +15), but the divine iato did almost the same dmg than the iato +15 (even when the AR of the divine is 100 higher).

Then i tried using magic arrorws in my pharis bow but the dmg is way weaker than using the cheapest arrows.

So, thw magic dmg is useful? there is something i'm doing wrong (choice weapon for example) or simply i'm attacking the wrong enemies?

There are different layers of defense on enemies. Some enemies are weaker versus different types of damage. When you hit a weapon that does both physical and magical damage, that weapon has to pass through all of the defense layers and there is a damage reduction with each layer.

That is why a weapon with a stronger base damage (for example..200/0/0/0) will often out damage a weapon that does 150/150/0/0. Because the 200 only has to go through 1 layer of defense, it will sometimes out damage the other weapon that is statistically stronger.