Ever wonder why your wedding invitation orders came with Tissue Paper?  The answer your question is as follows:  The type of printing presses used when printing invitations would leave the ink wet on the invitation, therefore, when assembling your invitation assemble, the tissue paper would be placed between the cards to prevent smearing.  With the new technology of today, this problem has been eliminated and the Tissue Paper is no longer necessary.  Most invitations no longer come with the Tissue Paper, but it is still available if you want to follow tradition.