What do 'Clear Spheres' in Final Fantasy X do?

Answer 1

They remove a non-skill sphere from the sphere grid. The primary purpose of this is to fill it with a better sphere. For example, removing a HP +200 sphere in order to add a HP +400 sphere. Removing an actived sphere causes it to be as if it was never actived, in the example's case, anyone who had activated the HP +200 sphere would lose 200 HP. However, as there are many blank places to begin with, this only becomes relevant in a either a completionist game, or in the International Version to help defeat the Dark Aeons. Clear spheres become available for purchase at the Monster Arena from the trainer, after unlocking Ultima Buster, for 10,000 gil apiece. Ultima Buster is unlocked by capturing at least 5 of every fiend in the game.