An early origin of best man at a wedding comes from Germany and Scotland. According to legend, villages and clans used to raid each other to gain brides. When there weren’t any available brides in a village or their allies, a young man would find a bride to be in an enemy family.
The young man would get a number of warriors, or grooms men to join him in his attempt to win the bride. These warriors would raid the enemy village and kidnap the chosen woman by force.
The best man, in this case the best warrior, would stay at the grooms side to fight off the woman’s family and help the groom escape with his bride.
Following this rough introduction the best man would help the couple hide from the woman’s family until the wedding day. This is why the Best Man stands to the right of the bride and groom, it allowed him to draw his sword and protect the important couple.
Source: Suite 101