Two advantages that immediately come to mind: 1) two-bar charged move, in contrast with Ampharos' existing options; 2) wide neutral coverage. Otherwise, it's almost as slow and easy to dodge as ZC and FB.
Unless Niantic is planning a buff to DP, or there's some kind of mega-update looming, Ampharos w/ this move is weird. Having said that, I'm not going to pass up this opportunity to stock up on as much Mareep candy as possible.