That means that every time a lawyer tries a case in court, the lawyer has the power to change the law. The ideas that may have originated with a faculty law review article or a law student law review article, but they get connected up with legal institutions through the mouth and the pen of the individual lawyer representing a real client in a particular case. So, it's not the judge that comes up with the idea for a new legal rule. It's the lawyer that wrote the complaint, who wrote the memorandum, who wrote the brief that gets accepted by the judge when it is persuasive. You don't have to be a litigator to change the law.
Clients come to lawyers because they want advice about important problems.
And at least some of the time they follow the advice. So, every time you advise a client, you have the power to change that client's behavior. And you don't have to be involved in an intimate personal relationship with a client to change things. Those lawyers are engineers, if you will. They are designing institutional apparatus that makes law and applies it.