Interpretation of grand potential.

Hello!

I understand Gibbs energy and Helmholtz energy, but how is grand potential to be interpreted and where is it useful?

For instance, Gibbs energy is the maximum amount of energy that can be used to do work at constant pressure-temperature while Helmholtz energy is the is the maximum amount of energy that can be used to do work at constant volume-temperature.

Does grand potential mean the amount of energy for a gas to exist, since it is defined as -PV? This is what it only means? Where is it useful?

Thanks!