Invalid patents are regarded as existing technology, and the rights and interests generated belong to all mankind. In other words, once the patent expires, anyone can use it for free to develop and sell products, and is no longer restricted by the rights of the original patentee.
The uselessness of invalid patents is reflected in the loss of the exclusive right to use technical solutions. Invalid patents can't be applied for any more, because novelty no longer exists. Therefore, other enterprises can use invalid patents to research and develop and produce products without worrying about licensing fees, which is an important way for enterprises to save R&D costs. Don't say "failure = uselessness", the wealth here is immeasurable.