I guess the point is that not all clothing is useful for every purpose or every occasion. Having clothing with specific traits to it and allowing you to choose the outfit that best suits your needs is the way to go. The biggest aspect they're adding is that sets give bonuses specific to a theme or role, with each set having different combinations of attributes. So a set of military style clothing would give you different attributes than clothing someone might use for parkour or skiing or mountain climbing. It's not just about temperature but precipitation, terrain, natural protections, needs for flexibility, durability, etc.