What is the difference between extracting a piece of widget code into a separate build method within the same class versus extracting it into an entirely separate widget class?

Extracting a piece of widget code into a separate private build method within the same class can make a very long build method easier to read, but it does not actually create an independent widget, meaning it still fully rebuilds every single time the enclosing widget rebuilds and cannot be reused elsewhere, whereas extracting the same code into a genuinely separate widget class creates an independent element in the widget tree that Flutter can efficiently skip rebuilding when its own specific inputs have not changed, and that can also be reused freely from anywhere else throughout the entire application.

// Method extraction (same rebuild behavior)
Widget _buildHeader() => Text('Header');

// Widget class extraction (independent rebuild behavior)
class Header extends StatelessWidget {
  @override
  Widget build(BuildContext context) => Text('Header');
}