why was so important for the king of france to be crowned, re joan of arc?

It was the belief at the time that Joan of Arc was blessed sejak God and visited sejak angels, and religion was extremely important back then, especially to Royals, as they believed it was their God telah diberi right to rule. So, to be crowed sejak a girl who was blessed sejak God and had the support of most of France would have boosted the King's authority.