Mock 2 Yield to Maturity vs Coupon on Debt

One of the accounting questions you had to caclulate the interest cost on outstanding debt. They used the YTM instead of the coupon rate. But why is this exactly?

The coupon rate represents the yield to maturity when their outstanding bonds were issued (assuming they were issued at par). The current yield to maturity is the rate that they would be able to borrow at today. If they issue bonds to finance capital expenditures now, they will pay the YTM, which is their pretax required return on debt.