Because the bond pays out at the end of the term, but you can sell it at any point of the term for whatever the market price is.

So for example let's say you bought a -0.1% yield, but newly issued bonds are now only offering a -0.5% yield you can now actually sell your bonds paying out -0.1% at a profit, since even if they paid you a 3.5% premium on the price you paid, it would still be more attractive than the rate on offer for new bonds.