May 17, 2012
Once upon a time, wedding invitations were delivered by hand. If you were a bride, your footman delivered your invitations to your guests’ homes. Their servant received the invitation and removed it from its mailing envelope (an envelope much too pedestrian for your guests to handle themselves).
The servant then presented the invitation to your guests in its pristine inside envelope. Because the invitation was already at its destination, the inside envelope had only the names of your guests written on it. The address was no longer needed. It just had to be directed to the appropriate members of the household.
Though footmen have since been replaced by postmen, the inside envelope is still de rigueur. It gives something as important as your wedding invitation a certain je ne sais quoi.
The White Crane - Fine Stationery, China, and Gifts