DAX Clarification

Following are 2 measures:

SUMX ( ALL ( SALES ) , SALES[AMT] )

CALCULATE ( SUMX ( SALES, SALES[AMT] ), ALL (SALES) )

Similarly for the following 2 measures:

SUMX ( FILTER ( SALES, SALES[QTY]>1 ), SALES[QTY] * SALES[AMT] )

CALCULATE ( SUMX ( SALES, SALES[QTY] * SALES[AMT] ), FILTER ( SALES, SALES[QTY]>1 ) )

Both above examples clear the natural filters on the SALES table and perform the aggregation.

what is the significance/usecase of using either approach.