Signatures that are witnessed hold up to closer scrutiny by the judicial system. By using witnessed signatures, parties ensure their intent is effectively communicated. Some types of documents must be notarized; that is the signatures on them must be witnessed by a notary public, reports Wikipedia.
LegalMatch.com reports that under the "statute of frauds" many legal contracts are required to be made in writing. It is mandated in many states that witnesses of signatures of wills not be designated as beneficiaries of the wills they are witnessing. Marriage contracts, deeds, mortgages and court documents are examples of documents where witnessed signatures are required.