I found this very interesting and thought I'd share it.
The tradition of a best man has its origin with the Germanic Goths, when it was customary and preferable for a man to marry a woman from within his own community. When women came into short supply "locally," eligible bachelors would have to seek out and capture a bride from a neighboring community. As you might guess, this was not a one-person operation, and so the future bridegroom would be accompanied by a male companion who would help. Our custom of the best man is a throwback to that two-man, strong-armed tactic, for, of course the future groom would select only the best man he knew to come along for such an important task.
The role of the best man evolved. By 200 A.D. his task was still more than just safeguarding the ring. There remained a real threat that the bride's family would attempt to forcibly obtain her return, so the best man remained at the groom's side throughout the marriage ceremony, alert and well-armed. He continued his duties after the ceremony by standing guard as sentry outside the newlywed's home. Much of this is German folklore, but is not without written documentation and physical artifacts. We have records that indicate that beneath the altars of many churches of early peoples (the Huns, Goths, Visigoths, and Vandals) there lay an arsenal of clubs, knives, and spears. The indication is that these were there to protect the groom from possible attack by the bride's family in an attempt to recapture her.
The link gives full details on the history of weddings and traditions in general. It's wonderfully fascinating!