Board Thread:Questions and Answers/@comment-28539895-20160524103442/@comment-28540184-20160524121127

Many years before the actual story begins, Aizen collected souls of many shinigami and offered it to his Hogyoku but it seemed not satisfied. So he stole the one from Urahara and offered it to his own to complete it. So basically a power increase, yes. Maybe, because the Hogyoku has its own will, it would have rejected Aizen in the first place if he had tryed to subdue it in an incomplete state.