In earlier times, a groom selected a close friend of his who stood guard in case some old suitor of the bride or competitor would try to steal the bride away before the ceremony was over. So he stood by the groom with his hand on his sword.
Today, the best man has a vital role at the wedding.
He is the Master of Ceremonies and Toastmaster of the reception and personal attendant and advisor to the groom.
He ensures the groom is dressed properly and at the ceremony site on time.
He takes care of the marriage license (that he signs), and the bride’s wedding ring: producing each at the appropriate time.
He takes the Marriage Officiant’s and the singer(s)’s/musician(s)’s fee from the groom and presents it each in a sealed envelope at an appropriate time.
He may also oversee the ushers, making sure they are properly dressed, understand their roles and are at the ceremony site in a timely fashion.
At the Reception:
During the reception, he proposes the toast that is usually a wish for health, happiness, and prosperity. He reads telegrams, makes introductions and any necessary announcements. He also sees to it that the reception moves along smoothly and that no practical jokes are played on the bride and groom. He is a “peace-keeper” of sorts.
After the Wedding:
His final responsibility is to help the newlyweds have a great start on their honeymoon. He assists the groom change into his travel clothes (he may take all the rental clothes back to the formal wear shop the following day) and may assist with last-minute packing. He ensures the luggage is safely locked in the honeymoon car or checked ahead at the airport or station. He may help the bride and groom into the honeymoon car or drive them to the airport. He should make certain everything is in order like passports, tickets, baggage claims and keys. The best man is responsibility for many things and should understand his duties so he can be prepared.
Makes plans for the bachelor party if he is giving one. Traditionally you should place the Marriage Officiant's fee in an envelope with the Officiant’s name on it and have the best man pay at a convenient time.