The world runs on trust
When I joined my sailing club in Hong Kong, my seconder, when signing my application in support of my membership, said to me "clubs run on trust."
He was talking about the fact that good clubs work and continue to exist because there is an element of trust among the members. There is trust that everyone will follow the bye-laws that they agreed to and trust that when they are making decisions for the club that they will put the club's and the membership's best interests before their own personal interests.
What's true of a sailing club is true of any organization large or small...from a couple to a family to a company to a nation. Groups and organizations that succeed and do well, do well because there's trust among the members and between the members and leadership.
Without trust we would need to clearly define or enumerate a precise set of rules that govern every minute interaction...and enforce them. The cost of this is too high. And it is impossible to write rules for a future that is impossible to predict.
Trust is what fills in the missing blanks between what we can define in advance and what we have to figure out on the fly. Trust that your fellow citizens and neighbors will be good citizens and good neighbors. Trust that your partner or spouse or children will act in the best interests of your relationship or family. Trust that your leaders will be make decisions in the best interest of the company and its employees or the nation and its citizens.
This is why is it so important to follow not only the of an agreement that binds an organization but also its spirit. To do otherwise harms trust. When trust breaks down, relationships fail, nations decline and companies stagnate and fail.