It shows employees and potential employees that you are a responsible and ethical company. If, say, your ethics are pro-diversity and anti-harassment, that makes you a more attractive company for which to work.
It shows your customers that you value integrity. If you live up to your code, they can do business with you and be confident that you will not cheat them to make a buck.
It tells your employees what you want from them. You may think the principles by which you run the company are intuitively obvious, but your employees' ethics may not be the same. Spelling out your code of conduct puts everyone on the same page.
Having a code spelling out unacceptable behavior helps employees to see that discipline is fair. If your code says your staff must be honest with vendors and customers, they will understand when you discipline employees who lie.
It tells employees what to expect from you and gives them a benchmark to judge whether you are living up to your own standards.
