This person is there for a number of reasons. HR gets paid to help companies be profitable by managing its personnel. HR tries to provide cost efficient and effective personnel to meet the company's goal for the bottom line. Personnel managers know if they fail to do what they were hired for that they will get fired. Sometimes it means they must tacitly ignore and violate the legal and ethical boundaries set forth in the employer-employee relationship.
Personnel often acts as judge and jury when it comes to employee related issues.
Their judgment calls on average are marginal at best. That means often they are wrong but the employee just doesn't know it. Too often management violates the law and personnel or HR ignores it. Other times is does not. Some HR managers in a corporate culture that will not tolerate management violating the law and side with the employee.