DirectSync was written to allow people to keep sets of information consistent between multiple Macs. It is not only a file/folder synchronization application (though its getting pretty good at that ;) ) nor is it a cloning/ghosting utility.
Instead it is focused primarily on "merging" data files for common applications such as Mail, iCal, Safari and potentially many others. The advantage of this approach over simple file synchronization is that certain types of files can be modified simultaneously and later merged together into a composite.
The developer did this because he found that he often forgot to use a file synchronization utility and by the time he remembered it was too late.