A more thorough explanation:

Useful, adjective. In the context of patents, it refers to an invention that has a practical application.

For an invention to be considered useful, it must have a clear and practical purpose that can be applied in industry. Mere curiosities or speculative ideas are not considered useful in the context of patents.

• A new type of machine that can efficiently sort and package products is considered useful because it has a practical application in the manufacturing industry.
• A device that can detect the presence of harmful chemicals in the air is useful because it can be used to protect people from exposure to dangerous substances.

These examples illustrate the concept of usefulness in patents. Both inventions have a practical application that can be used in industry or to protect people from harm. They are not mere curiosities or speculative ideas, but rather have a clear and practical purpose.