Whetver missions the planet might have given the weapons are irrelevant, because Sephiroth is controlling them, by the time that you see any of them. They are freeing up mako to return to the lifestream, so Sephiroth can absorb it and become more powerful. The Ultimate Weapon may have originally been created to protect the planet from Jenova, but that's definitely not its mission as it appears in the game.