Not sure about round tripping but perhaps the longer words might stress test webpage layouts.
For German, I'd assume it's just about the special characters (äöüß), ie testing that encodings are somewhat correct (at least beyond ASCII).
It will do a good job of pointing out places where you haven't built in adequate flexibility for longer words. It's not perfect of course, there are times when a translated word is shorter, but it is a nice real world test of the basic flexibility of your page layout.