The superstition that it is bad luck for a groom to see his bride prior to the wedding ceremony on the wedding day can be traced back to a time when arranged marriages were common; the bride would be kept from the groom prior to the ceremony to ensure the groom wouldn't back out of the arrangement after seeing the bride. The concern was that a wealthy groom might find the bride unattractive and call the wedding off, which would be detrimental to the bride's family. Though arranged weddings are no longer common and marriages are supposed to be more for love than money, many engaged couples decide to adhere to this tradition to enhance a sense of excitement and anticipation prior to their wedding ceremony.
The 'don't let the groom see the bride before the ceremony' tradition isn't the only holdover from a bygone era of business transaction weddings. The bridal veil is another vestige of this practice. Not only was the groom prevented from seeing the bride prior to the ceremony, but her face would be kept covered until vows had been exchanged in order to prevent a last minute change of heart by a groom who figured he could probably do better.