Even though there are armor sets, having a full set besides cosmetic purposes doesn’t offer anything else given that all mesh armor sets have similar resistances and the same goes for cloth, leather and plate.
Since each armor has its own description, it would be nice if there was a corresponding effect for the set. For example the verdant set could have a stamina regeneration effect, the bane armor could have damage reflection and poise, the plague doctor set could have plague resistance up and fire dmg up and so on.
I believe that would make compulsive hoarders such as myself invest even more time in collecting everything the game has to offer.