Plugins, or goodies I think ddg calls them keeps you within the site and provides more info in that zero-click box.
The in part is they get submitted and we try hard to make them all useful and thus "in" DDG by default. However, ultimately you could a) consume them in your own applications (off DDG); and b) choose to turn some off or on more of the time.
That would be a good set of options however.