Normally, wedding guests wouldn't dream of dressing like the bride on her big day. However, bridesmaids were originally instructed to dress exactly like the bride so that she could hide among the lookalikes. This would confuse any evil spirits that entered the church, thus leaving the soon-to-be married couple safe to say, "I do."
In addition to throwing off the evil spirits, the matching outfits would have also confused any jealous men or angry family members who were intent on stealing the bride. The safeguard against bride thievery is also why the woman stands to the left of her future husband during the ceremony: the groom and his best man needed to be able to fight off intruders if necessary.
Eventually, the tradition of matching outfits evolved to have the bridesmaids wear shorter veils with their white dresses during the Victorian era. Today's bridesmaids wear dresses that match in color, style or both, but they typically do not mimic the bride's dress.