Didn't understand where to use Void function and Return type function

Didn't understand where to use Void function and Return type function

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I am really new to programming and while learning Dart function I come to a program which has but I couldn't understand what makes them to use different function one with return value and another without. Both will called during main program and both subprograms complete the task without returning any value(at least I didn't see anything like that), Can anyone point out why one have to use return type function.

import (....)

void main() => runApp(XylophoneApp());

class XylophoneApp extends StatelessWidget {

//Void Fuction
  void player(int songNumber) {
    final player = AudioCache();

//Function with return
  Expanded buildKey({colors, songNumber}) {
    return Expanded(
      child: FlatButton(
        color: colors,
        onPressed: () {

//Call in main function
buildKey(colors: Colors.red, songNumber: 1),

The XylophoneApp.player function doesn't return any value since it's a void function (Instead it will just play a sound), where the XylophoneApp.buildKey function will return a value of type Expanded, then it's up to you whether you want to use the returned value or not.

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The method player simply plays a song and returns back to the caller, i.e doesn't return any value to the caller method. Hence return type is Void.

Whereas, the buildKey method creates a Button which needs to be rendered in the UI. buildKey method is creating a widget that is a FlatButton, which when clicked, plays a song. This method returns this button which needs to be placed in the Widget hierarchy for flutter to render it on screen.

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