Why is temperature & pressure constrained in a saturation state?
Given the 4 main properties - Temperature, Pressure, Volume & Internal Energy (with the others derived from these), is is such that in a saturation state, temperature & pressure are constrained, such that, e.g., saturated water & steam (and any quality in between) at a particular temperature corresponds to a particular pressure - and vice versa. However, the volume & internal energy (and all the other properties) are different for saturated water & steam (and for any quality in between.) It seems that there must be a reason for this.
Perhaps the reason is that for material to be in thermodynamic equilibrium, there cannot be internal work done or heat transferred. And as work is only done if there is pressure difference - and heat transferred is there is a temperature difference - for a saturation state to be in equilibrium, the material in the 2 phases must be at the same temperature & pressure,
There probably is a better explanation, but I think I have the gist of it here.