Edit: changed answer to make it suit to question as it is formulated for now.
In most RPG games I had experience with, armor is used for three purposes (sometimes combined):
1) It is effective way to make character gear/equipment to affect character somehow. Same to weapon increasing damage character deals, armor decreases damage character takes.
2) Equipment may grant different stat bonuses/penalties, having different pros and cons. Player may have to choose, what to prioritize - armor for physical protection (by choosing armor with high armor stat), attack speed (by choosing light-weight armor), or stats providing.
3) Some RPGs may have more complex damage/armor system, providing different kinds of damage types, each one having corresponding protection stat. Armor in this system usually represents physical damage resistance. By varying resistance stats of enemies, developer may create different situations for player - some battles will feature monsters with high physical damage resistance/armor, but will be vulnerable to magic; player will have to focus on magic skills instead of plain attacking in such situations. It also can be combined with p.2., allowing player to focus on defense against certain damage type before next battle.
Anyway, this is mostly used ways, but developer is always free to decide, how to use armor stat.