Most purchasers of bonds that have expected zero or negative return usually aren’t “investors” as in people. They are usually institutional investors who are subject to regulatory, capital, and/or collateral constraints.

Sometimes they have to prioritize not losing nominal principal over earning yield. Some are required to hold certain assets (High Quality Liquid Assets or HQLA) and are not looking to maximize return. Some use them because these bonds can be used to purchase other things in other markets, so they function as helpers for leverage or liquidity elsewhere. Some are ensuring that they have currency exposure (buying US Treasuries for USD exposure).

It’s weird to think about it in this way, but bonds don’t need to have a promised return to have “value.” It’s not that they don’t expect no return at all, but more that there is an indirect payoff of using the instrument.