Debt subject to this kind of bail-in would supplement the increased regulatory capital in order to provide greater assurance that, should the firm  limited, though not prohibited, rehypothecation of customer securities. Other countries may try to limit the practice informally, but have no formal rules.
It can provide liquidity through open market operations by purchasing assets in the open market in exchange for newly created reserves. It can also address the liquidity needs of solvent but temporarily illiquid banks by lending to them through the discount window.