The ADC measurement is always relative – relative to your voltage reference.
The most of the applications don't allow to use accurate and expensive voltage reference. In that case, VDDA is used as the reference for ADC measurement.
Since the operational VSUP range starts from 3.5V and accuracy of the voltage regulator is limited, we should use internal bandgap reference for compensating ADC voltage results. The bandgap voltage VBG has a narrow variation over temperature and external voltage supply.
The example shows how to compensate ADC results by additional bandgap voltage measurement.