Question about an episode in Mrte D'arthur

I was really interested in the brief episode in the Morte in which Balin and the damosel came to the castle and the damosel had to bleed into a silver dish in order to help the lady get well. I thought it was an interesting episode, but I didn't really understand why it was included in the Morte. Of course, blood as tribute for passage, is present in many stories, the fact that the blood had to be from a princess in order to be effective was an interesting twist. Why does it have to be royal blood? What does this passage mean to the rest of the text?