DDL-defined views offer quite a few features that are not possible with DDS, and the iSeries Navigator interface makes them easy to use.
These features include the ability to define a view of a view, more complex join logic, summary details, more complex derived columns, and more complex row selection.
One of the more interesting capabilities of DDL is the ability to define a view of a view, or to define a logical view of a logical view. Why would you ever want to do this? A few reasons come to mind.
You may want to define multiple views, in which each view selects the same subset of columns but has different criteria for selecting rows. You define a view that selects the subset of columns, and then define different views ...