Many skills give a character % damage, including many passives, buffs, link skills, and all the Reinforce hyper skills for attack, which just give % damage for that particular skill.
The reason % att is better is because it's much rarer. Without any of either, adding say, 12% attack is the about the same as adding 12% damage, though % damage is applied after % attack, and % attack is applied to your attack and rounded down, so % attack could be slightly worse. They're both additive on top of themselves and multiplicative with each other.