The U.S. Patent and Trademark Office cannot be aware of all prior art in a particular field of technology at the time a patent application is filed in that field. Therefore, the patent applicant is required to file an Information Disclosure Statement when the patent application is filed, or shortly thereafter, that discloses the prior art that is most relevant to the claimed invention.
The inventor is more likely to know the prior art in the field of invention than the USPTO so the burden is on the inventor to educate the USPTO about the prior art. Failure to comply with this requirement can invalidate any patent that issues on the grounds that the inventor committed a fraud on the USPTO by not disclosing prior art of which the inventor was aware.