For Radarr, it's for cleaner separation and easier automation. Also, keep in mind that you cannot have the same movie with two qualities on the same instance.
My 4K instance has one root path (different from non-4K) and one quality profile that's auto-selected by default. Using the Radarr connect option, I have a quality profile on the non-4K instance that automatically adds the movie to the 4K instance. This way, you can ensure you have both 4K and non-4K copies of certain movies (e.g., for external access/transcoding). You also ~never need to actually interact with the 4K instance. See this page for details: https://trash-guides.info/Radarr/Tips/Sync-2-radarr-sonarr/
For Sonarr, it's also for separation, but less of an issue if you're on Sonarr v4. The one thing you gain even on v4 is the ability to have different quality definitions for anime and non-anime content. See: https://trash-guides.info/Sonarr/Sonarr-Quality-Settings-Fil...
My 4K instance has one root path (different from non-4K) and one quality profile that's auto-selected by default. Using the Radarr connect option, I have a quality profile on the non-4K instance that automatically adds the movie to the 4K instance. This way, you can ensure you have both 4K and non-4K copies of certain movies (e.g., for external access/transcoding). You also ~never need to actually interact with the 4K instance. See this page for details: https://trash-guides.info/Radarr/Tips/Sync-2-radarr-sonarr/
For Sonarr, it's also for separation, but less of an issue if you're on Sonarr v4. The one thing you gain even on v4 is the ability to have different quality definitions for anime and non-anime content. See: https://trash-guides.info/Sonarr/Sonarr-Quality-Settings-Fil...