The stack determines who bears losses and in what order, and it is the frame for capital structure arbitrage: buying one layer and shorting another when their prices imply inconsistent views of enterprise value.
A useful discipline is to price the whole company once and then allocate. If the bonds imply a 60% recovery and the equity still carries a meaningful market value, one of the two is wrong, because equity is only worth something when debt is money good.
Example: enterprise value $1.2 billion, secured debt $500 million, senior unsecured $600 million, equity market cap $30 million. Unsecured holders recover $700m / $600m, so they are money good and the equity stub is a thin residual claim.
Related: seniority, subordinated-debt, recovery-rate, distressed-debt, corporate-bond