Usefulness or "utility" is one of the three cornerstones of patentability under U.S. law (the others being novelty and non-obviousness). An invention must be useful to be patentable. At a minimum, the invention must be operable as described and achieve some purpose. It must solve a technical problem. The asserted utility must be credible to a person skilled in the art. Inventions definitely lack utility if they are inoperable (e.g., involve perpetual motion), frivolous or fraudulent.
If a person skilled in the art to which the invention pertains would doubt the utility of the claimed invention as it is disclosed in a patent application, then the invention may be unpatentable. In such situations, the applicant must be able to offer evidence to rebut the challenge.
Law - Useful