What does agency theory study? Why is this more important in listed companies than in private enterprises?

The agency theory studies why enterprises have agents, because the directors and shareholders of listed companies are not only one company, but often hold several positions, so agents appear, which can manage the company more systematically and lead the company to the next level. Why is this more important in listed companies than in private enterprises? Because a company has more than one shareholder or director, the agency in a listed company is more important than that in a private company.