The obvious answer is that in ESO, armor is the fulcrum of the offense/defense trade-off. One that is necessary in any RPG.
But I will say that it never made much sense to me that armor should provide any offensive buffs at all. It's not like the type of clothes you're wearing is making your daggers sharper, or your blows more penetrating. From a point of view that is consistent with the logic of the game world, I would expect armor bonuses to be wholly defensive in nature, possibly some in regards to regeneration/encumberance.
I would expect heavy armor to have more defense in return for limited movement; I would expect medium armor to provide better dodge rolls, e.g. longer invincibility frames; I would expect magic-focused light armor to be more receptive to enchantments. But I wouldn't have expected medium armor to directly boost my weapon damage. That's just weird.