I understand why you would want to set interest rates negative if your central bank is going to buy up all that debt, but as an individual or institutional investor why would you ever want to hold something with negative interest instead of cash? Why is there even a market for a bond with negative interest rates at all, since compared to a $X bond with negative interest, $X in currency seemingly carries less risk, mo…
However, sometimes they have more money than their investment prospects can handle. Normally under those circumstances, they would buy treasuries from the federal reserve, which return the "risk-free rate". They always want to invest their money in things that return more than the risk-free rate, but sometimes they can't find enough stuff to invest in that they think will have a better return. In those cases, they're forced to buy treasuries.
Now, if the risk-free rate is negative, you might reasonably ask: Why doesn't the bank just keep cash? And the answer to that is that they're legally prevented from doing so. Banks have to maintain their capital reserves at the Fed, and the Fed pays them the "federal funds rate" on those reserves. So if they can't lend out the capital, they have to keep it with the Fed, where it earns whatever rate the Fed chooses to give them.