For example, making an AOT compiler. Or adding other features. Or making the implementation more suited to client side apps. There are many things you can do to a language to make it easier for a specific use case. In the extreme, this is what DSLs are.
I don't understand why you don't understand that having first class control over a language's development is a useful thing to have.
All else being equal, it is still useful to have control over a language's development. It doesn't matter that Dart may or may not have features not present in other languages, as long as they can add them as necessary based on Flutter's development.
I don't understand why you don't understand that having first class control over a language's development is a useful thing to have.