How and why did France unite around the king?

In the Middle Ages, people united around the king, because he was the guarantor of the observance of their rights. The medieval monarch provided support to all segments of the population and gave them protection. The unification around the king took place at a rapid pace, because a single power rallied the people. The medieval ruler had a professional squad, which prevented the ruin of farms and robberies. After the unification of the territories, they had to pay tax to the treasury, but this did not prevent the expansion of the territory of the central government.