What you mean by "professional institution" is not quite clear. I am going to assume you mean an organization that you might join voluntarily, for example, a professional organization such as the American Management Association or the Chamber of Commerce. A professional institution might very well mean an organization that you "join" by virtue of employment, or an institution that you "belong" to by means of certification or licensure, for example, by becoming a Certified Public Accountant. Given any of these interpretations of this term, there are good reasons to be subject to a professional code of conduct. In fact, why would we not want everyone to be subject to a professional code of conduct?
In any business enterprise, we want people to behave with professional and ethical conduct. This allows for fair competition, a somewhat level playing field. A capitalistic society that does not adhere to these standards turns into a Darwinian "survival of the fittest" society, antithetical to the Western democracy, which has the stated goal of equal opportunity for all.
As consumers, we want to deal with people who act professionally and ethically. Do you want to patronize a business that cheats its customers? Would you want to use an accountant who did not get continuing education in accounting? Do you want to do business with a company that rapes the environment?
A business might succeed for a while without such conduct, but sooner or later, it will pay the price. Examples include Enron, Martha Stewart, and the current Murdoch scandal. Unethical and unprofessional business persons suffer unpleasant consequences, for example, the loss of business, the close of a business, fines, or imprisonment.
Most organizations today, whether they be for-profit or non-profit, have a code of conduct to which you are expected to adhere as an employee. I have provided a link to one such code below. A failure to adhere to such codes results in discipline, demotion, suspension, or loss of employment. Not joining a professional organization does not insulate anyone from these consequences.
As professionals of any sort, whether or not we join an organization with a code of conduct, we should all subject ourselves to such a code, for our own good and for the good of the world in which we live.
Joining a professional organization and subscribing to its Code of Conduct is an indication that one considers ones self bound by the highest standards of the profession. This can lead to increasing comfort among those with whom one does business. I am not sure anyone wishes to do business with a physician who does not subscribe to the code of ethics of the AMA, or an attorney who proudly avoids the canons of the ABA. Every profession has standards, and professional associations set those standares. One would do well to avoid those who refuse to subscribe to them.