The final quest in Wind Waker before getting to Ganon's Castle, is the Triforce Quest. The Master Sword has its power back, but now Ganon is already in Hyrule, searching for Zelda, and could've already found her. To get back to Hyrule again Link needs to find the 8 pieces of the Triforce of Courage.
The first time Hyrule opened up, Link had just completed the trial of the gods, and the gods allowed him enter Hyrule in order to get the Master Sword. Then, after he saves Aryll from the Forsaken Fortress, he goes into Hyrule again with Tetra, and the King of Hyrule reveals that Tetra is Zelda and that she has Triforce of Wisdom. Why is it just now that you need the Triforce of Courage to go back there again?
It seems that going into Hyrule requires either the approval of the gods, or the ownership of one of the 3 triforce parts. This explains why Ganon can enter Hyrule at will since he has the Triforce of Power. When you entered with Tetra, she had the last missing piece of the Triforce of Wisdom.
Link's triforce quest had already started even in the middle of the 2 sage awakening quests. The 8 triforce pieces are scattered across the entire overworld, and finding them all without a guide requires you to explore nearly every square of this massive Great Sea.
Treasures are hidden all over this huge ocean. Some might contain rupees, sidequest items, heart pieces, or a treasure chart that marks one of those items on the map... But some treasures might contain one of the 8 triforce charts, which are needed to find the 8 shards of the triforce of courage.