I want to put the subtitles where I want and pick the font, I want to be able to apply image filters to my movie (I use a monitor calibrated for photo, and it is not ideal for movies, it is much easier to apply a filter than to switch my monitor calibration), also I watch things while working and mpv is much lighter than any browser, I love to be able to seek with the mouse wheel, the stereo mixing on netflix is often horrible and I can't hear the dialogues... There are tons of argument in favor of a real video player.