Correct me if I'm wrong, but I'm pretty sure that casting your screen re-encodes the screen image in much lower quality.
Whereas Airflow is sending the original video steam over untouched, for supported codecs like h.264/265. So it maintains the original quality. Including resolutions that may be higher than your screen, like 4K.
Using the screen over AirPlay is the wrong way to go for watching video, for several reasons.
That said, QuickTime Player also supports AirPlay to cast the video to an AirPlay device. I’m struggling to understand why I’d use AirFlow when I can use QuickTime? In my case I use Plex, which even makes QuickTime a moot point. I’ve also used VLC with the VLC AppleTV app; I could see where AirFlow could be an improvement over the rather clunky VLC experience (from what I remember).
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.
Whereas Airflow is sending the original video steam over untouched, for supported codecs like h.264/265. So it maintains the original quality. Including resolutions that may be higher than your screen, like 4K.