Drops from enemies are used in ascending weapons, characters and talent levels. Genshin Impact heavily relies on farming in-game materials and requires players to consistently farm materials to progress. Drops from mitachurls can be used in different characters. As you progress through the game, even drops rarity will matter to ascend higher rarity items. The rule of thumb is to collect anything that shines; you never know which material you may require down the line, and running out of material will be bad. I always keep my inventory stacked up, just in case I decide to level up a character or a weapon in the future.