Reversible Work

Some work always gets wasted as heat. In other words, all of the work that is done on a body cannot get stored as internal energy--some of it leaks out as heat. The amount of wasted work is minimized when a process is carried out reversibly. A reversible process happens very slowly and the system is always in equilibrium (i.e., the intensive variables are uniform) during a reversible process.

This idea of a minimum of something is very useful because it allows the definition of new thermodynamic variables associated with the limiting (minimizing) case. This is an abstraction--like the notion of limits in calculus--which seems confusing at first, but becomes natural with familiarity.