The tradition has three aspects:

1) It reminds the groom that his new wife is valuable and that he needs to watch out for her safety.

2) It protected the bride from the "ius primae noctis". In this variation, the best man is doing the groom a favor, by hiding his wife from the nobility.

3) It's a historic relic, reminding us of theft-marriage and Beutefrau (war brides).