It reduces the effort of learning different frameworks for the frontend and the backend. You use one language that includes simple interoperatbility between the frontend and the backend. It will still obviously need to be compiled into a format that browsers can work with but if you're gonna use a framework anyway then Dart/Flutter is a reasonable choice (modulo performance issues as mentioned by others in this thread).
The point is no one uses plain javascript anymore so if you're going to use some domain specific language with its own build pipeline to generate compiled assets for the browser then Dart/Flutter is no worse than any other javascript framework.
