While I’m a firm believer in the notion that people who are willing to engage in unethical behavior regardless of their codification, I also see that honor codes are absolutely necessary in most universities.
First, universities, especially R1 institutions, are multicultural communities. That is, they are assemblies of a multitude of perspectives and ethical outlooks. Individuals with different outlooks on research and success see value in doing good work to ensure success, or doing whatever work creates that success. I highlight this because it can exist as both a cultural difference and a personal perspective. In the midst of a university, the complex nuances of ethical research need to become clear, through a unified code of ethical conduct.
Second, honor codes deter those who may be tempted by the easy road to success that dishonorable conduct offers. Cheating, plagiarism, and falsification are relatively easy to do, and they seem just as easy to implement without consequence. Those who find themselves on the verge of dishonest work may find clarity in an honor code, and in that clarity, a deterrent to those actions.
For these reasons, an honor code is a necessary component in a university’s efforts to prevent unethical actions in its community. Universities realize that unethical behavior damage their reputations, and by proxy that of its many alumni. Any action that the university can take to discourage breaches in ethics will benefit the whole.