How is HBOGO different from Xfinity On Demand? If you subscribe to HBO, all content is anyway available OD, so there is no real need for a separate 'client' for watching it.
Last I checked, all the content wasn't available on-demand either. There was a larger selection on HBO Go versus On Demand. OD only has the last 1-2 seasons of the series it has at all, and has fewer movies. HBO Go has all the seasons of all the series.