"Share" is one of the worst inventions of all. What it does in phones is random across apps and platforms, and usually has nothing to do with what the word "share" means in any other context.
If you think of it as "send" rather than "share" it makes a lot more conceptual sense. Don't get caught up on the word.
It's almost always to send the content somewhere, whether it's a platform, an app, the clipboard, etc.
Not always always, but almost always.
You're sharing data between apps. It's an app->app API, essentially. You can easily send an app store listing to your Reminders "Wishlist" section if you want, for example.
It's definitely not only social sharing.