For certain documents, it is important to verify that the person signing the document actually signed the document. A notary acts in that case like a professional witness. In some situations, a non-notary witness will suffice, but for certain important documents it may be necessary to have a notary act as a witness.
Some documents even require the swearing of an oath, or the making of a solemn declaration. For those documents, a notary is qualified to administer the oath or declaration. If the person making the oath or declaration gives false evidence in the document, he or she may be found guilty of perjury, which is a criminal offence.
A notary can also certify that a copied document is a true copy of an original document. In order to do this, the notary must see the original document to confirm that the copy is indeed a true copy. In these cases, the notary is not typically confirming that the contents of the document itself are accurate, but simply that the copy is an accurate copy of the original.