For another perspective: USD are "hoarded" by investing them in US treasuries. This preserves their value against inflation, because (to a first approximation)

For another perspective: USD are "hoarded" by investing them in US treasuries. This preserves their value against inflation, because (to a first approximation) when the Fed prints money it does so by buying those treasuries, thus raising their price in USD. So in the dollar economy, hoarding takes the form of purchasing USD so you can participate in seigniorage. In the bitcoin economy, hoarding takes the form of holding on to your balance. But they are both forms of hoarding, designed to protect value without actually making "useful" investments that affect the real world.