Why use a return inside a sumx

GuillaumePower I agree that the construction of the measure is odd and complex. However, without additional information regarding the model, the intended use case, etc. it's hard to know why it was constructed the way it was. It may be that there is a good reason for it or it may be that it could be done much more simply and efficiently. 

You can use nested VAR statements in a measure, for example, you could write this:

Sales PQ =
VAR __Result = 
IF (
    [ShowValueForDates],
    SUMX (
        VALUES ( 'Date'[Fiscal Year Quarter Number] ),
        VAR CurrentFiscalYearQuarterNumber = 'Date'[Fiscal Year Quarter Number]
        VAR DaysSelected =
            CALCULATETABLE (
                VALUES ( 'Date'[Day of Fiscal Quarter Number] ),
                REMOVEFILTERS (
                    'Date'[Working Day],
                    'Date'[Day of Week],
                    'Date'[Day of Week Number]
                ),
                'Date'[DateWithSales] = TRUE
            )
        RETURN
            CALCULATE (
                [Sales Amount],
                'Date'[Fiscal Year Quarter Number] = CurrentFiscalYearQuarterNumber - 1,
                DaysSelected,
                ALLEXCEPT ( 'Date', 'Date'[Working Day], 'Date'[Day of Week] )
            )
    )
)
RETURN
  __Result

There are simpler ways to get a previous quarter calculation, such as the following examples:

Internet Sales (PQ) = 
    CALCULATE([Internet Sales],
        FILTER(ALL('Dates'),
            'Dates'[Date] >= MIN('Dates'[Prior Quarter Date]) && 
            'Dates'[Date] <= MAX('Dates'[Prior Quarter Date])))

TI PQ 2 = 
    CALCULATE(
        [Internet Sales],
        DATESBETWEEN('Dates'[Date], MIN('Dates'[Prior Quarter Date]), MAX('Dates'[Prior Quarter Date]))
    )



NC Intenet Sales (PQ) = 
    VAR __MinPYDate = MIN('Dates'[Prior Quarter Date])
    VAR __MaxPYDate = MAX('Dates'[Prior Quarter Date])
    VAR __PQCalendar = CALENDAR(__MinPYDate, __MaxPYDate)
    VAR __Table = 
        SUMMARIZE(
            ALL('FactInternetSales'),
            'FactInternetSales'[OrderDate],
            "__Sales", SUM('FactInternetSales'[SalesAmount]))
    VAR __Result = SUMX( FILTER( __Table, [OrderDate] IN __PQCalendar), [__Sales] )
RETURN
    __Result