> but as an individual or institutional investor why would you ever want to hold something with negative interest instead of cash?

There are no effective ways besides these bonds to hold tens or hundreds of millions of dollars as cash, even if you hire a warehouse, security, and store pallets of fiat (which has been attempted). You are paying a premium for principal safety.

You either make this choice voluntarily as an asset manager, or because you are required by regulation or other means to hold a percentage of your portfolio in this asset class.