User blog comment:A.J AL/Season six plot hole./@comment-24169020-20121215024922/@comment-3481504-20121217000454

Angels and demons alike can absorb souls, in the same fashion as Castiel did in "The Man Who Knew Too Much" and "Meet The New Boss". By being the King of Hell, Crowley would've been able to utilize the souls there to boost power his power.