The Gibbs energy is used because most experimental data have been determined at known temperature and pressure and any other thermodynamic quantities can be calculated from it.
The discrepancy between model and reality is finally represented by a power series expansion in temperature, pressure and constitution of the phase.
The strength of the CALPHAD method is that the descriptions of the constituent sub-systems can be combined to describe a multi-component system.
The second crucial factor is the availability of computer software for calculating equilibria and various kinds of diagrams and databases with the stored assessed information.
They are used in research and industrial development (e.g., PrecipiCalc software and Materials by Design Technology), where they save large amounts of time and resources by reducing the experimental work and by making thermodynamic predictions available for multi-component systems that would be practically unattainable without this approach.