LMS Teacher

My left side tires, both front & rear were cupping on the inside only on my 06 LS430 after 42K miles. Took car into dealer for tires and he performed a 4 wheel alignment and said I needed new rear camber bars both right and left sides. Although the right side tires seemed ok wear wise with 42K on the tires, service mgr said both camber bars needed to be replaced. What are they and why would a bar go bad?