After the moving flashback we just had it feels good seeing Law take all the emotions he has for Rosinante and channel it through the purpose he inherited from him i.e. to stop Doflamingo. Doflamingo may ultimately be Luffy’s opponent, but having the focus centered around Law and Doflamingo for the moment does serve the purpose of increasing the weight the fateful day thirteen years ago had, for both Law and Doflamingo. Law is using memories of that day to focus his wrath while Doflamingo is becoming unsettled by the day he would like to forget.
The Ope Ope no Mi, the light of salvation at the end of the darkness gripping the very life of a young boy victim to the greed of minds unconcerned for any interest outside their own. A legendary Devil Fruit capable of performing “miracles” and the only solution available to Law for curing his Amber Lead Sickness. Rosinante, aware of the state Law is in and the lack of time available to him, resolves to go against both the Donquixote Pirates and the Marines, in order to secure the Ope Ope no Mi and save Law.
So there is a certain “Royal Treasure” hidden away somewhere in Mariejois which the World Government, or more accurately the Tenryuubito, would do anything to keep from being found out by the world. For that reason once Doflamingo had come to find out about the existence of such a “treasure”, the Tenryuubito used everything they could to try and kill Doflamingo. But with Doflamingo being too strong and able for them, they opted to cooperate with him instead, hence why even with Doflamingo attempting to destroy everything the Tenryuubito controls around the world, they still ended up giving him a hand when he needed it earlier in this Dressrosa Arc. In addition, Doflamingo intends to use that “Royal Treasure” to obtain the “treasure” of this world, but for that he needs Law’s Ope Ope no Mi.