The original duty of a "Best Man" was to serve as armed backup for the groom in case he had to resort to kidnapping his intended bride away from disapproving parents. The "best" part of that title refers to his skill with a sword, should the need arise. (You wouldn't want to take the "just okay" member of your weapon-wielding posse with you to steal yourself a wife, would you?)
The best man stands guard next to the groom right up through the exchange of vows (and later, outside the newlyweds' bedroom door), just in case anyone should attack or if a non-acquiescent bride should try to make a run for it.
It's said that feisty groups like the Huns, Goths and Visigoths took so many brides by force that they kept a cache of weapons stored beneath the floorboards of churches for convenience. Modern-day best men are more likely to store an emergency six-pack at the ceremony for convenience, but the title remains an apt one...
Historically, the bride's dress was actually selected with the purpose of tricking the eye of evil spirits and jealous ex-lovers! The Bride's Bridesmaids and faithful attendants were instructed to wear a dress similar to that of the bride so that during their group stroll to the church it would be hard for any ill-willed spirits or former boy-toys to spot the bride and curse/kidnap/throw rocks at her. (Same for the men in matching suits, saving the groom from a similar fate.)
With all that Life has to offer,