For an invention to receive a patent in the United States, it must meet certain characteristics. Among these, it is required that the invention be "useful." If it is not deemed useful by the patent examiner at the U.S. Patent and Trademark Office (USPTO), the inventor will be denied a patent.
"Useful" is an interesting word, since at first blush, it feels subjective. An invention that is "useful" for one person might not be "useful" for another. What exactly does "useful" mean in the context of a U.S. patent application?
The requirement that inventions be "useful" in order to be eligible for patent protection has its basis in both the U.S. Constitution and the Patent Act.
Under Article I, Section 8, Clause 8 of the Constitution, Congress shall "promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries." The "useful arts" has been interpreted to mean that proposed inventions must have utility. This creates a minimum constitutional threshold for a patent.
Similarly, the Patent Act (at 35 U.S.C. § 101) says that someone who "invents or discovers any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent therefor, subject to the conditions and requirements of this title." As you can see, the word "useful" appears twice in this gateway provision of the Patent Act; any patent application must propose a "useful" invention. This requirement, sometimes called the "utility requirement," applies to utility patents, the most common type of patent issued by the USPTO. Through this language, Congress incorporated the Constitution's mandate into its patent legislation.
Essentially, the proposed subject matter for the patent must be immediately useful. It cannot be merely theoretically useful. Thus, a proposed patent on the chemical for a cleaning fluid that might be an effective laundry detergent, after undergoing further research and development, is not eligible for a patent because it is not yet useful.
Importantly, this does not mean the invention necessarily needs to work perfectly—for example, a patent on the formula for laundry detergent need not remove every single stain—but it must generally do what the patent claims.
For an invention to be patentable, it does not need to be the most efficient, effective, or perfected form of a particular use (whether it is a medication, a piece of software code, or a machine). It merely needs to work as described. Some patent scholars have described this threshold has having a minimal level of social benefit.
Clearly, this is not meant to be an incredibly high bar. Most inventors would not bother patenting an invention (especially given the costs and time involved in the process) unless there was a fairly clear use for it, whether on its own or as part of a larger product or configuration.
Inventors should usually describe the utility of the invention on the face of their patent application, so as to ensure that they overcome this initial hurdle in obtaining a patent. Failure to do so can result in the USPTO's examiner denying the application.