Although there was the danger that local lords would pursue their own interests and create chaos and instability, the feudal system was also capable of being the foundation for powerful states. This is why the system lasted so long. It was simply better than any other system at the time for controlling large areas of territory.
We must remember that this was a time when communications and travel were very slow. This meant that it would be very hard for a king in a central location to govern and to protect a large area of territory. This was shown, for example, by the kings’ inability to prevent the Viking attacks that were such a nuisance to so many areas of Europe. Because it was so hard for a central government to rule, it was important to have a system of decentralized government. This was where the lord-vassal relationship came in.
The lord-vassal relationship created a system of obligation that ran from the king all the way down to the lowest levels of society. Many people took these bonds of obligation very seriously. Therefore, many lords did try to do things that would help the country as a whole and many kings tried hard to help the lords. The lord-vassal relationship gave these people reasons to help one another and, thereby, to maintain a coherent society.
Without the lord-vassal relationship, it would have been hard for any central government to rule a large territory. This relationship made it possible for strong countries to emerge because it created a system that encouraged cooperation between various people in society, thus allowing large territories to cooperate in a coherent way.