I do this because it lets me have each video in its own resizeable window, scrub around much more quickly using keyboard/mouse shortcuts, customize many aspects

I do this because it lets me have each video in its own resizeable window, scrub around much more quickly using keyboard/mouse shortcuts, customize many aspects of playback like subtitle styling ,brightness, hue, even flip or rotate the video if I want, set the volume on a per-video basis even >100%, save exactly where I was in a video and how it was set up, not have every video I see a part of show up in my history (even hovering for preview does that now, annoyingly), and install a multitude of plugins to augment the player even further. Though, I initially did start using mpv for watching videos back in 2014 when I was stuck with a core2duo laptop, and it was the only way of watching 720p or higher video without slowing the machine to a crawl. I'm so used to this setup now that I wouldn't want to go back to having my videos trapped a browser tab. (or an always-on-top firefox popout window)