An RSS datasource can be usefull to get notified when something gets updated.
But for a website like MangaDex it's probably less useful. Because MD has the follow overview.
But some might still use it to get a notification when a specific manga has a new chapter.
Personally, I only use it to keep track on specific light novels.
As example, here's a screenshot of an RSS reader app showing the latest updates based on the RSS link for Solo Leveling.