The phrase "in witness whereof" in a legal document is used to identify someone who witnesses the execution of the document, according to TheLawDictionary.org. A witness is generally not a party to the document's content or agreement.
Many legal documents require one or more witnesses at their execution. A last will and testament carries more weight in court if signed before witnesses, notes TheLawDictionary.org. A notary public is someone granted authority by a government to serve as an official witness. Notaries are granted a license and must meet certain requirements to qualify. Not all witnesses are notaries, but all notaries are witnesses.