Why have code of ethics become so common?

It defines what is expected of employees, officers and members of a company or a organization. By putting it in writing and asking every person to acknowledge it the organization is saying this is the standard they expect. It also makes it possible to make some public relation points by being able to say, "this is what we are and how we act". A third reason is that by publishing the code it makes it easier to take action when a member of the organization violates that code.