Fundamental Equations and Relations (also known as Equations of State, the most famous being the Ideal Gas Law: PV=NT (We're taking the energy definition of temperature here, otherwise there'd be a k)) are *VERY* important. Mainly because with all Equations of State, you can determine EVERYTHING about the system!

An Equation of State is an equation relating an intensive parameter in terms of a set of extensive parameters. I.e.:

T = T(S,V,N)

P = P(S,V,N)

μ = μ(S,V,N)

This is mainly here to just lay down the concept, but other than that, it isn't needed computationally.