They helped to break down feudalism by increasing the authority of kings.
Some nobles died in battle without leaving an heir. Their lands passed to the king.
Some nobles sold their land in an effort to raise money to pay the special tax levied by the king to offset the cost of the crusades.
Some nobles gave their serfs a chance to buy their freedom in an effort to raise money they needed to buy armor and weapons
Some young men who could buy their way out of feudal obligation joined the crusades. Many died. That reduced the work force. If a farm failed, it passed to the king.