Ah, I missed that Airflow was doing transcoding, that sounds like the primary benefit, and one of the big things I use Plex for.
I will day, video formats seem to be much more standard (and compatible with QuickTime) than they were in the pre-h.264 days. Though I do tend to lean toward mp4 over mkv if given an option. I still have a second player around to handle stuff QuickTime fails on, but I only need it a few times per year.
Most h.264/265 files in the wild, QuickTime won't play. You have to repackage them with a couple of specific flags using ffmpeg.
QuickTime is great for playing videos produced by Apple devices, but terrible at playing media generally.