Who and why was interested in centralization and strong royal power?

After the unification of the state, the royal power became the only center of government for the country. Such states, which were governed from one center, were called centralized. During the late Middle Ages, such states arose in Spain, England and France. In them, large feudal lords swore allegiance to their king. The knights, who were subordinate to the lords, were now transferred to the service of the king. Thus, all feudal lords in these countries, from large to small, obeyed the king.