Backward measurement of AVcc against reference voltage connected to ADC input
An ADC reads an analogue value via its input but that analogue value is dependent on: -


The analogue value
The ADC's reference voltage


If one of the two above analogue voltages are known the other can be computed or inferred.

I wouldn't have thought it was very common because most folk will want to have a stable voltage reference (from a known voltage reference chip) so that they can make accurate voltage measurements of the various input signals going into their ADC (multiple channels are of course usually "served" by one stable reference).

However, if the inputs are a bunch of RTDs and the RTDs are excited via a resistor from the AVcc rail then ratiometric measurements are the most accurate method of using the ADC and, if AVcc is also needed to be known then a stable voltage reference on a spare ADC input allows you to make this measurement.