I use Lemmy Community Seeder. Every four hours it checks the top posts on instances you specifies and automatically subscribes you to communities that appear there but you aren’t already subscribed to. You can tweak it to ignore specific communities or instances.
In addition to the reasons already mentioned, Apple has a requirement that applications have a novel component. While it’s often questionable as to what is considered “novel” Weather applications get contrasted against the built-in weather app. If the app simply duplicates the functionality it will be rejected.