The fundamental reason we use Dart is simply that it is the language used to develop for Flutter. The reason we use Flutter is that it is a framework developed and supported by Google for
"building beautiful, natively compiled, multi-platform applications from a single codebase"
The Flutter platform provides Dart classes called Widgets that are used to describe elements that are rendered in the user interface. Beyond these, Dart is a statically typed language that is also used to write any other classes your application requires.