why is this working?

SELECT Sale_Day, Item_ID, Total
FROM
(
SELECT Sale_Day, Item_ID,
       SUM(Rate*Quantity) Total,
       RANK() OVER(PARTITION BY Sale_Day ORDER BY SUM(Rate*Quantity) DESC) rank
  FROM scratch
 GROUP BY Sale_Day, Item_ID
) a
 WHERE rank = 1
ORDER BY Sale_Day