I may be misunderstanding, but don’t this involve an intermediary re-encoding when casting video from phone → dongle? If so that’s unavoidable quality loss which isn’t great and runs down phone battery unnecessarily. If it works for you though, that’s what’s important!

Aside from that, having a little extra power available can be nice for being able to smoothly play back video formats that the hardware doesn’t have acceleration for, obviating need for transcoding (via Plex or similar).