Why was joan of arc important in the hundred years war?

Because she somehow managed to turn the tide of war in the favor of France. After she did this the French were able to win the war though it did drag on to some degree for decades after she was killed by the English.

She was an important figure in that she is considered a hero for role in the Hundred Years' War. She is considered a saint because she heard the voice of God that told her she would lead the French to victory but died burned at the stake at age nineteen. She was a true leader and is not a catholic saint