Before the postal system was founded, invitations were delivered by men on foot. Often their hands were dirty or soiled and thus, so was the mail they were delivering! This led to wedding invitations being delivered in two envelopes so that the invitation and its envelope remained clean and protected. Although the mail is no longer delivered by foot (though sometimes it feels this way!), there are still benefits to using an inner envelope with your wedding invitation set.
It adds a layer of protection in the mail.
Your envelope liner will have more impact because the inner envelope has no gum and won’t rip when opened. You can also print a message detail on the shoulder of the envelope.
It allows you an opportunity to specify if any children or guests are invited.
It is an added cost for the second envelope and for the calligraphy required as both your inner and outer envelopes will need to be calligraphed.
It is an additional step to assemble your invitation suite and will require a bit more time and attention if you are stuffing and mailing yourself.