Firstly, it’s unheard of to do this after the cabinet is appointed but many PMs have done this in the first days after an election as a temporary measure.
Secondly the politics are obvious – his minister had granted a mining exploration lease off Sydney’s northern beaches, which left all his Sydney MPs scrambling to convince him that this would be electoral suicide…so Morrison appointed himself to overturn the decision. The mining company is now suing the government claiming that the lease was improperly overturned…the court case will be another cost we taxpayers don’t need
Why he wanted powers across the other portfolios remains to be discovered…