It's burned as fuel in a gas turbine to generate electricity. The process is similar to burning natural gas in turbines, but some equipment adjustments have to be made when running on hydrogen. The Delta project is starting with a 30/70 blend of hydrogen and natural gas, gradually stepping up to 100% hydrogen.
Siemens currently has turbines that can burn up to 75% hydrogen and is targeting 100% hydrogen by 2030:
No it's mostly the chemical energy in the stored hydrogen, pressure is only a small part of it (though they might insert some expander turbine somewhere in there). Pretty similar to how you'd rate a natural gas storage facility.
It should be noted that the energy stored in the pressure of gas is zero. All the energy of an ideal gas is the kinetic energy (and rotational/vibrational energy) of its molecules, which is thermal energy. Internal energy of an ideal gas is independent of pressure.
What compressed gas storage does is (aside from combustion energy of a fuel gas) to provide not a store of energy, but a store of reduced entropy, enabling otherwise unusable amounts of heat to be converted to work.
