Deferred imports in Dart

deferred-imports.dart

// Importing standart math library
// Note that with deferred imports we have to specify its name
// using _is_ keyword
import 'dart:math' deferred as math; 

void main() {
  greet();
}

// This is a function that is async. I will talk about
// these kind of functions later on
Future greet() async {
  // Actually loading the library
  await math.loadLibrary();
  // Using it
  print(math.sin(0));
}

What is this deferred keyword does?
Why is it required since dart is compile time language, I don't quite get the concept of delayed or dynamic import on dartlang