Thermodynamic state equations (e.g. the ideal gas law) are most commonly used in systems involving a gaseous phase. They are usually used to convert from one variable to another. For example, we may be interested in modelling how the pressure changes in a gas vessel during a fault condition. We can't do a pressure pressure balance (it isn't a conserved quantity), but we can do a mole balance and then convert the change in the number of moles into a corresponding pressure change using a state equation.