Why must you use Intent.setAction and Intent.AddCategory to use singleTop with PendingIntent?

Question

I'm using example code to create a notification, which I then want to handle in onNewIntent() if my App is already running. However, onNewIntent() was not being called. I searched for a few hours and no one seemed to have a proper answer--just some workarounds.

I had to search forever to find the solution here: Android OnNewIntent not called and the answer is not actually explained.

Question

Can anyone explain why we need the lines:

resultIntent.setAction(Intent.ACTION_MAIN);
resultIntent.addCategory(Intent.CATEGORY_LAUNCHER);

in order to receive the notification through onNewIntent()? The App launches just fine without them, but will always go through onCreate() instead.

Example Code

 public void createNotification(String s){
        // The id of the channel.
        String CHANNEL_ID = "my_channel_01";
        NotificationCompat.Builder mBuilder =
                (NotificationCompat.Builder) new NotificationCompat.Builder(this)
                        .setSmallIcon(R.drawable.icon_missing)
                        .setContentTitle(getString(R.string.notification_channel_name))
                        .setContentText(R.string.text);
// Creates an explicit intent for an Activity in your app
        Intent resultIntent = new Intent(this, MainActivity.class);
        resultIntent.setAction(Intent.ACTION_MAIN);
        resultIntent.addCategory(Intent.CATEGORY_LAUNCHER);

// The stack builder object will contain an artificial back stack for the
// started Activity.
// This ensures that navigating backward from the Activity leads out of
// your app to the Home screen.
        TaskStackBuilder stackBuilder = TaskStackBuilder.create(this);

// Adds the back stack for the Intent (but not the Intent itself)
        stackBuilder.addParentStack(MainActivity.class);

// Adds the Intent that starts the Activity to the top of the stack
        stackBuilder.addNextIntent(resultIntent);
        PendingIntent resultPendingIntent =
                stackBuilder.getPendingIntent(
                        0,
                        PendingIntent.FLAG_UPDATE_CURRENT
                );
        mBuilder.setContentIntent(resultPendingIntent);
        NotificationManager mNotificationManager =
                (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

// mNotificationId is a unique integer your app uses to identify the
// notification. For example, to cancel the notification, you can pass its ID
// number to NotificationManager.cancel().
        Notification n = mBuilder.build();
        n.flags = n.flags | Notification.FLAG_AUTO_CANCEL;
        mNotificationManager.notify(1, n);
    }