Value the business as if all-equity, then add the financing benefit separately: V_u = FCF₁ / (k_u − g) plus PV(tax shield) = T·D gives APV = V_u + PV(shield). The shield is the tax saved on deductible interest. More debt means a larger shield, so long as you ignore distress costs.
With no debt the gold block disappears and APV collapses to V_u. Push the debt slider up and watch the gold shield grow. In reality the rising chance of financial distress eventually offsets the shield, so more debt is not free value.
Reflect: APV adds the shield as a separate term, while WACC lowers the discount rate to bake the shield in. For a perpetual fixed debt level the PV of the shield is simply T·D, discounted at the cost of debt k_d. If the firm instead rebalances debt to a constant ratio, the shield carries the business risk and is discounted at k_u. Which convention fits a real company better, a fixed debt schedule or a target leverage ratio?