Why would a labor union need all workers to agree to join?

A labor union typically requires all workers to agree to join to ensure collective bargaining power and solidarity among members. When all workers participate, it strengthens the union's negotiating position with employers, as it represents a unified front. Additionally, universal membership helps prevent free-riding, where some workers benefit from union gains without contributing to its efforts. This collective agreement fosters a sense of shared responsibility and commitment to common goals.