maybe don't look at it from vader's view, but from krennic? he is desperate for this thing to work, he thinks it is his ride to advancement and he wants the emperors approval. instead of the emperor he is stuck with trying to impress tarkin and then gets summoned to see vader. so it could be as much about showing krennic never being able to get an audience with the emperor or that he has to keep going little bits of the way before he will be allowed the priveledge to see him?