Why is the technology of hydrogen fuel cells significant?

Because they are a renewable energy source and do not involve any CO2 emission.

When hydrogen and oxygen are combined in the fuel cell the only waste product is water so technically the fuel cell is non-polluting. The oxygen needed can be obtained from the air. The challenge is that there is no source of natural elemental hydrogen gas on earth, so the hydrogen gas has to be made, usually by the electrolysis of water. This requires large amounts of energy so we can only consider the hydrogen fuel cell to be renewable if we have a readily available source of renewable energy to make the hydrogen gas in the first place.