This Could Save Your Job
One of the most vital functions of a union steward is to prevent management from intimidating employees. Nowhere is this more important than in closed-door meetings when supervisors or guards, often trained in interrogation techniques, attempt to coerce employees into confessing to wrongdoing.
The rights of employees to the presence of union representatives during investigatory interviews was announced by the U.S. Supreme Court in 1975 in the National Labor Relations Board vs. J. Weingarten, Inc. Since that case involved a clerk being investigated by the Weingarten Company, these rights have become known as Weingarten Rights.
The Weingarten Rights say that if an employee requests a steward before or during the interview, the employer must grant the request or end the interview.
The employer is not obligated to inform you of these rights. It's up to the employee to request union representation and refuse to answer any questions until a steward is present.
Source: The Legal Rights of Union Stewards