The history behind it is that when raised printing methods were first invented, the inks they used were still wet when they came off the press.
The tissue paper was used to blot the ink and prevent it from smudging .... and also allowed the invitations to be stacked without ruining the back of the top card.
It doesn't help to protect them in the mailing process.
Most of the modern invitation companies (including the majority of the higher end designers) do not use the tissue paper any longer.