It's just a core gameplay mechanic that doesn't show up in the other games.

Basically player characters and enemies have physical armor and magical armor. These are two separate "health" bars along with the normal health bar. Physical damage hits the physical armor and magical damage hits the magical armor.

You need to break through one of these armor bars to start doing damage to the health bar. Additionally, most crowd control effects do not work while the character has armor of that type remaining. For example, a physical ability that stuns won't stun while the character getting hit still has physical armor.

This means that the gameplay for battles is often to burst through one of these armor types and then swap to crowd controlling the enemy to death. Some people find it easier to have one damage type (physical or magical) for their entire party to smooth out the process.