QwtPointSeriesData what for?

[IwantToLearen]

I would like to know what is the benefit of using QwtPointSeriesData class.

f.e.

QwtPointSeriesData  mySeriesData;
QwtPlotCurve            myCurve;
QVector<QPointF>    myVector;
 
  /////1 
  mySeriesData.setSamples(myVector);
   QPCruve.setData(mySeriesData);
 
//and 
////2 
QPCruve.setSamples(myVector);

QwtPointSeriesData  mySeriesData;
QwtPlotCurve            myCurve;
QVector<QPointF>    myVector;

  /////1 
  mySeriesData.setSamples(myVector);
   QPCruve.setData(mySeriesData);

//and 
////2 
QPCruve.setSamples(myVector);

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what is the difference between 1 and 2.