Nice question, but I think the answer is a little unclear.
The important thing to note is that the parentheses of the example do not have anything to do with the DISTINCT keyword. This should become more obvious with the following example:
SELECT DISTINCT
NULL Dummy
,(datepart(year,ModifiedDate)) as year
,(ModifiedDate)
,(1) + (2)
from Person.Contact
order by ModifiedDate
These parentheses are valid, but also superfluous as they provide no additional value.
Best Regards,
Chris Büttner