The bayeux tapestry is a historic document why

The reason why its a document because the Bayeux Tapestry gives information of a historical nature that no other source has provided. It begins with the description of Harold's eventful journey to Normandy (landing in Ponthieu, meeting with the Duke William, expedition in Brittany and oath on the sacred relics of Bayeux). It then depicts Harold's return to England and his coronation after the death of King Edward the Confessor. Finally it describes the preparation for William's expedition, the crossing of the Channel, and the Battle of Hastings.
The story, as described in the Bayeux Tapestry is broadly in keeping with the account made by authors of the 11th century : William of Jumièges, William of Poitiers and Orderic Vitalis. William Caillou,a monk at the abbey of Jumièges, wrote in 1060-1070 a history of the Dukes of Normandy, which ends with an account of the conquest of England. William of Poitiers, one of the Duke's knights, who then became his chaplain, wrote around 1070-1075 a biography of William, which ends just after the conquest. As for Orderic Vitalis, he was born in England in 1075, of an English mother and a French father who fought in Hastings. He arrived in Normandy in 1085, became a onk at the monastery of Saint-Evroult (Orne) and spent much of his life compiling a voluminous History of the Normans, an important part of which is devoted to William the Conqueror. One should also mention the poem written by Guy of Amiens, chaplain of Queen Matilda, which is an epic account of the different phases of the Battle of Hastings.
Compared to these written sources, the Bayeux Tapestry offers original information, found nowhere else, particularly with regards to civil and military architecture, weapons, navigation and elements of everyday life.