A prerequisite for grouping the result rows is a specific sort of the result table. Such a sorting must be requested within the SELECT statement (by ORDER BY). REPORT cannot perform the sort afterwards.
On the one hand, grouping improves the clearness of the report, because the individual groups are separated from each other by space lines and recurring values only appear at the end of a group; on the other hand, it is a prerequisite for the calculation of subtotals (SUBTOTAL).
See also:
Additional Functions for TOTAL and SUBTOTAL