What is the main advantage of using Data Labels in PowerPoint to label your data series, as opposed to relying on the Y$-$Axis?

Data Labels always look more professional.

Data Labels are more accurate as they tell you exactly what the values are, as opposed to having to look back and forth at the Y$-$Axis to guesstimate the values.

Data Labels are easier to format than a Y$-$Axis $($and you can clone your data labels whereas you cannot clone a Y$-$Axis$).$

None. The best practice for labeling charts in PowerPoint is to use both Data Labels and the Y$-$Axis.