As holder of the office of earl marshal, the duke was the country's senior nobleman.
From the time.com
The 1st duke left two sons, of whom Thomas the elder was only recognized as earl marshal.
From the en.wikipedia.org
More agreeable was his duty as earl marshal, choreographing the annual opening of Parliament by the monarch, a ceremony largely unchanged since medieval times.
From the economist.com
Each of William's sons did become earl of Pembroke and marshal of England, and each died without issue.
From the en.wikipedia.org
Marshal did not obtain Pembroke and the title of earl, which his father-in-law had enjoyed, until 1199, as it had been taken into the king's hand in 1154.