Why do invites have inner envelopes?

First let me start by saying, this is the most commonly asked question by ALL of my Brides and Grooms when it comes to the details of their invitation. Each time I explain why there is a need for an inner envelope, but also keep in mind... it's a new era and people do what they want, not everyone feels it's necessary to have an inner envelope. With that said, I, however, follow the rules of Emily Post Wedding Etiquette, and she says just this:

The inner envelope has the title and last names of the Guest(s) receiving the invite and nothing more, no address on this envelope. This also allows the host to specify who is invited and who is NOT invited. It is also ok to write familiar names here like; Nana and PopPop or Uncle James and Aunt Jean. If you are only inviting adults then the inner envelope would look like this:

Mr. and Mrs. Oliver Smith

If you are inviting their children along with the parents, the inner envelope would read:

Mr. and Mrs. Oliver Smith
Jennifer Smith
Brian Smith

The inner envelope is another place for where to write "and Guest". If you are guesting someone with their invite, it's always nice to include the Guests' name if you know it, but if not "Guest" is fine. It is not appropriate to write "and Guest" on the OUTER envelope, which is reserved for addressing purposes, hence the inner envelope.

Last but not least, this allows you another area of your invite to color coordinate with your wedding theme by lining the inner envelope with a color of your choice! You're wedding invite is once in a lifetime... go all out!