Multiple instances are needed if you want multiple versions of a given movie or episode. For example, I have a Dolby Vision device that works great with certain files, but on some other devices the file works poorly and doesn't transcode well. Plex handles multiple files fine.

Definitely not a usecase for everyone

The arrs have extensive and solid capabilities for filtering for the quality you want, but they don’t really offer the ability to manage multiple qualities for one specific item.

It’s also nice to have them separated because you can set up distinct plex libraries mapped to the sonarr and radarr 4K media directories. I don’t give everyone access to 4K content because no matter how many times I explain direct play, some people just seem to love transcoding to 720p. So I can limit 4K library access on plex and disable transcoding altogether for plex content.

I also don’t want every item I add in both 4K and 1080p.

Obviously the instance is to hide away from wife, kids and rest of the family. I recommend stashapp.