Workers' compensation covers a sub's own employees if they are hurt at work, paying medical costs and a portion of lost wages regardless of fault. In most states it is mandatory for businesses with employees.
If a sub lacks workers' comp, an injured worker's claim can roll uphill to the general contractor's policy, and the GC's own premium audit may charge for that uninsured labor. Verifying WC, or a valid exemption, is essential risk management.