Ordered sets are powerful features, but are not widely regarded as such and not widely known in the developer community. The idea is actually quite simple: data is grouped normally and then the data inside each group is ordered given a certain condition. The calculation is then performed on this sorted data.
A classic example would be the calculation of the median.
One way to get the median is to take sorted data and move 50% into the dataset. This is an example of what the WITHIN GROUP clause will ask PostgreSQL ...