Ordered sets are fairly new to PostgreSQL. The difference between an ordered set and a normal aggregate is that in the case of an ordered set, the way data is fed to the aggregate does make a difference. Suppose you want to find a trend in your data—the order of the data is relevant.

Here is a simple example of calculating a median value:

test=# SELECT id % 2, 
         percentile_disc(0.5) WITHIN GROUP (ORDER BY id) 
   FROM generate_series(1, 123) AS id 
   GROUP BY 1; 
 ?column? | percentile_disc 
----------+----------------- 
        0 | 62 
        1 | 61 
(2 rows) 

The median can only be determined if there is sorted input.