What does "ORDER BY (SELECT NULL)" mean?

The following SQL is from Itzik Ben-Gan that is used to generate a numbers table. What does the order by (select null) part mean? Thanks.

DECLARE @number_of_numbers INT;
SELECT @number_of_numbers = 100000;

WITH    a AS ( SELECT   1 AS i
               UNION ALL
               SELECT   1
             ),
        b AS ( SELECT   1 AS i
               FROM     a AS x ,
                        a AS y
             ),
        c AS ( SELECT   1 AS i
               FROM     b AS x ,
                        b AS y
             ),
        d AS ( SELECT   1 AS i
               FROM     c AS x ,
                        c AS y
             ),
        e AS ( SELECT   1 AS i
               FROM     d AS x ,
                        d AS y
             ),
        f AS ( SELECT   1 AS i
               FROM     e AS x ,
                        e AS y
             ),
        numbers
          AS ( SELECT TOP ( @number_of_numbers )
                        ROW_NUMBER() OVER ( ORDER BY ( SELECT   NULL
                                                     ) ) AS number
               FROM     f
             )
    SELECT  *
    FROM    numbers;

Thanks!