From TheGamer: “For those who want to fully 100% Nier Replicant, which includes upgrading all weapons, completing all quests, and discovering every single ending farming for materials is going to be a core part of your gameplay. Even players who don’t plan on going the whole hog will find themselves needing to track down various materials for quests as part of their normal playthrough.”

Source: N4G PC Nier Replicant: Where To Find Eagle Eggs