Is invalid patent useful or useless?

It's no use. It is useless to prove patents. An invalid patent generally refers to a patent that has been declared invalid but has not been approved, or a patent that has lost its patent right for various reasons stipulated by law and is no longer protected by the patent law. Invalidated patents become a kind of free social public resources, and naturally become "well-known public resources" because of their expiration. Invalid patented products are no longer protected. Since the invalid patent has been patented and made public, the re-submitted patent application is not novel. However, an invention-creation that is improved on the basis of the original patent may be granted a patent right after applying for a patent.