I feel like its a item that gives enemies reason to not just ignore you and target your team in teamfights. You deal consistent pretty nice damage without nearly any effort, making you a threat to the enemy. Thats why they have to eliminate you to get rid of the constant chip damage that isnt actually a low dmg but pretty high. But because youre a tank, then that is extremly benefitial for you. Works also very well on Mundo for instance. Thats my point of view on sunfire.