Build it because it is huge and you can blow up anything with it including my father's shed. I also want it to replace the moon as it will be fascinating to see a huge steel ball orbiting earth knowing it is capable of turning your homework into dust with its huge laser beam thingy :):):):)
One, it would have no use, as humanity doesn't need to wipe out any other planets. Two, the costs would be tremendous. It would cost trillions if not quadrillions of dollars to build this, and it has no purpose. Three, humanity isn't advanced enough to build a massive floating station equipped with plasma guns and supplies.