It's mentioned in the 2nd bullet point of the article link: 23.5MWh/m3 The benefit is you can store energy, in aluminum. It has a high energy density, you can

It's mentioned in the 2nd bullet point of the article link: 23.5MWh/m3 The benefit is you can store energy, in aluminum. It has a high energy density, you can safely store it anywhere, it requires no containment at all, etc. Your last statement includes the answer to your own question. They are using the hydrogen, from water reacting with aluminum, to power a fuel cell.