It is a perfect staging area because the downside to attacking it is so high. No allied forces in Europe want to deal with the fallout - literally.
This is so genius I'm genuinely in awe. This is right out of Sun Tzu: “The art of war teaches us to rely not on the likelihood of the enemy's not coming, but on our own readiness to receive him; not on the chance of his not attacking, but rather on the fact that we have made our position unassailable. ”