A more thorough explanation:

Usefulness, also known as utility, is a requirement for patentability. It means that the invention must be useful for something.

For example, a new type of phone case that protects the phone from water damage would be considered useful because it helps people protect their phones from water damage.

The Patent and Trademark Office has guidelines for determining compliance with the utility requirement. The invention must have a credible, specific, and substantial utility. This means that the invention must have a real-world use that is supported by logic and facts, specific to the subject matter claimed, and not require further research to identify or confirm a use.