What is the importance of royal blood to King Leodogran in Idylls of the King?

No amount of bravery, no nobility short of royalty, no political power can possibly equal being the child of a king. Leodogran does seem to be satisfied with Bellicent's story, however, which portrays Arthur as the son of no one, not even Uther, but as a child miraculously borne in by the ocean.