I agree it sounds a lot like homework....
however, .. yes it matters.
The reason is SQL presents the results grouped (nested) in the order you asked, so SALES grouped by yr, mth would group sales by yr, then by mth within that yr. Evidently the other way round would make no sense unless you were looking for seasonal patterns...
Because it's easier to code group by internally by first sorting the data as per request, group by normally implies order by the same values will occur in the output.