Every group have a tendency to build its own core set of rules which in the first place encompass the fundamental reasons of existance of that group. In nations we see this applied with constitutions. These rules sets are normally more abstract and profound than any other laws and they tend to express the purest form of social forms and creeds of the group.
A constitution inspires its citizens and is there as a solid guideline for the conduct of the individuals as well as the political forces in a group.
Still these rules have a life cycle made of relatively small and rare amendments. It is probably even more vital that changes to the constitutional laws are under the scrutiny of the entire population with the aim of guaranteeing that the group protect itself from abuses or from sudden changes in policy clashing with those fundamental principles agreed upon the constitution of the group itself.
Having constitutional law as a ‘senior citizen’ in the overall system helps in the purpose of having a more stable and peaceful evolution of the law of the group.