The beginning of the Magna Carta is basically a list of the important figures that were there when it was written.

It begins with:

JOHN, by the grace of God King of England, Lord of Ireland, Duke of Normandy and Aquitaine, and Count of Anjou, to his archbishops, bishops, abbots, earls, barons, justices, foresters, sheriffs, stewards, servants, and to all his officials and loyal subjects, Greeting. (preamble.1)

…and it continues by naming others present at Runnymede.

This is before the custom of signing documents at the bottom, and these dudes wanted to make sure they got their names in somewhere, so they began by listing them all at the top including all their titles.

It's a bit like when teachers ask students to head their papers. It's so that everybody will know right away who wrote this magnificent work of wonder—or so that they know who to blame.