Because when you differ, you want to know the difference in energy between two states that feature different thermodynamic conditions. The A is energy given a particular N,V,T tuple. That means that A actually depend on that tuple and when you want to compute the difference in A between two different NVT tuples you have to define A as a function of NVT:

dA = -pdV + SdT + Xdmu

Because when you differ, you want to know the difference in energy between two states that feature different thermodynamic conditions. The A is energy given a particular N,V,T tuple. That means that A actually depend on that tuple and when you want to compute the difference in A between two different NVT tuples you have to define A as a function of NVT:

$$\mathrm dA = -\,p~\mathrm dV + S~\mathrm dT + X~\mathrm d\mu$$