Held individually the negative-yield bonds don't make much sense. However, they can actually improve the risk-adjusted returns of a portfolio that also holds stocks. This is because long-term bonds have, in the past decade, been negatively correlated with stocks [1,2].

[1] https://imgur.com/a/r9nCsN5

[2] https://www.portfoliovisualizer.com/asset-correlations?s=y&s...