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.