What property of hydrogen makes it useful as a fuel

In terms of energy density, liquid hydrogen requires much more volume than other fuels to store the same amount of energy. Four liters of liquid hydrogen are needed to match the same energy content of one liter of gasoline. On the other hand it is also one of the lightest fuels available; 1 liter of hydrogen weighs only 0.07 kg. That is a density of 70.8 kg/m³ (at 20 K).