I think of it the other way around: Why expand the bond assets to issuers in other currencies?
Having bonds exposed mostly to CHF is a natural thing to do because the role of bonds in a portfolio is to dampen fluctuations, with low volatility and liquidity. For the growth part, I let the stocks do the job and rebalance regularly.
But the number of issuers in CHF is limited and this means a concentration risk. By expanding to USD, EUR and other currencies, I also expand to other governments and companies, which reduces that issuer concentration. Still, to preserve the CHF exposure, I hedge.
Hedging only protects against short term and middle term fluctuations. The graph above goes over 36 years, but on such large periods, the effect of hedging is negligible (the swap rates fully absorb the interest rate difference, and at that scale the evolution of the currencies is mostly the inflation difference: in real terms it’s a flat line).