why does she tell them that they will be victorious ? what is her purpose?

Although she is a female and is not seen to have the strength and fortitude of a man, she assures her troops that she has the courage of a King and this courage is doubled because an English monarch has more heart than a ruler presiding over another nation. She is as indignant at the prospect of a European invasion as any male monarch would be and condemns this invasion believing it is the very fact that she is a woman that has emboldened these foreign invaders. Rather than allow an invasion to succeed she will take to the battleground herself, preferring to die in the glory of defending her realm than to be captured. Troops that valiantly fight alongside her will be rewarded.