Hi,
The major difference between Aggregation and projection node is , by adding a column as 'Add as aggregated column' you can do the aggregation.
But in projection you can just add a column as output .
So if you add a aggregation node in calculation view and if you want to do aggregation on any measure , you need to add that measure column as 'Add as Aggregated Column'.
If you just add that column as 'Output' , In data preview , you will not see any aggregation done on result.
In projection , you only have option to add column as Output.
Regards
Vishal Jadhav