Because using this expression in the front end, may cause slow performance. I mean it can load much more time to the selected value.
If I make it as a dimension in the script itself, it is pre-calculated and no loading issues will be there.
Please check the below expressions whether it will take long time to load,
Count({<Customer=E(Customer),WithProp_ID=P(WithProp_ID)>}FieldName)
sum(AGGR(if(Count(DISTINCT TOTAL <WithProp_ID> Customer)>1,1,0),Customer,WithProp_ID))
