Some armors give small bonuses and no drawbacks. Other armors give big bonuses, but with drawbacks to balance them out. The obvious example is Chaos Cloak/Cowl, which offers massive offense but also status penalties. (Arguably the penalties on Chaos should be bigger, but that's another debate.) The Plate sets are supposed to offer massive defense, at the expense of attack speed. They're the closest we get to a "tank" archetype in this game --- slow, but invulnerable.