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 :):):):)
It doesn't just wipe out planets, people. We could pinpoint any military enemy of the UN and wipe it out in a brilliant wave of destruction. No more terrorists, no more rogue nations, no more dictators. The US would have constant surveillance over Earth, and the ability to destroy anything that violates their rules. Not to mention, it would be an excellent way to make contact with extraterrestrials when the time comes to do so.
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.