One very important function of an armor is the ability to change the character's element (level one always). In all cases except for Ghost, wearing the particular element armor will reduce damage from that element, which is absolutely necessary to reduce an MVP's elemental skill, and also even for some regular monsters. Each element has a card for it, but certain armors such as Lucius's Fierce Armor of Volcano  also change a character's element by default.
Note: do NOT card an elemental armor such as a Lucius's Fierce Armor of Volcano  (fire property) with another elemental card (like a Swordfish Card for water). The card will OVERWRITE the existing element with the card's element (fire overwritten by water).
Reference: Elemental Armor
Aside from element change, armors can also offer increased HP, increased ATK and MATK, and other small bonuses.
Armor Level 2
|Item List||Obtained From||Level||DEF / MDEF||Description|
|Anti Conjurer Suit ||Geffen Night Arena||230||120 / 0|
|Magic Protector Robe ||Geffen Night Arena||230||110 / 0|