The purpose of thermodynamics

Thermodynamics is presented mostly as an equilibrium theory because equilibrium state is the simplest case. But from the very beginning, thermodynamics was developed to understand non-equilibrium problems. Carnot studied the heat flow between two sources of different temperatures. Each of the two sources is in equilibrium state. But the heat flows between two sources of different temperatures are non-equilibrium processes.  

The purpose of the thermodynamic theory is very much like the purpose of calculus. Differentiation is linearization. But the main purpose of calculus is to do integration, a nonlinear process, through the infinitesimal changes of linearization. Similarly, the main purpose of thermodynamics is to understand non-equilibrium processes, such as the working of engines, through the infinitesimal changes of equilibrium states.

However, equilibrium thermodynamics itself is an extremely complex theory. Most people get stuck there and rarely escape it. Hence most researchers think thermodynamics first and foremost as an equilibrium theory. This situation is what we hope to change.