Useful energy can be converted into some kind of work, while useless energy is useless. One of the contributions of Josiah Willard Gibbs was to formalize these concepts, by introducing the concept of “free energy.” Schrödinger didn’t use that term in his lectures because he worried that the connotations were confusing: The energy isn’t really “free” in the sense that you can get it for nothing; it’s “free” in the sense that it’s available to be used for some purpose.
