Banks (and other financial institutions). They more or less have to in order to make the books add up; or, in the clearing process, leave their money with another bank, most likely the central bank, which will pay little or no interest.

If you try to leave your money with the Swiss Central Bank they will charge you 0.75%. Buying bonds is definitely preferable to that. https://www.snb.ch/en/ifor/finmkt/operat/id/finmkt_nz