This document helps establish the essential fact that you are intending to marry within the country you are traveling in and have legal evidence that you are not currently married. Please note - in some cases, this is not a government document, but rather this is an affidavit that is signed in the presence of a Notary Public. This sworn-affidavit confirms that you are who you claim to be and are free to marry.
In the past, the United States Embassy granted Americans the right to swear to an affidavit of entitlement to marry in front of a Consular Office. However, in recent times, due to local administratiors, officials interpreted these records to signify that the United States Embassy had verified the content of the individual's testimony But no evidence was provided. Almost all foreign countries require that a couple provide a Single Status Affidavit of some kind before they will be granted a Marriage License.