Somewhat related anecdotal tale:
There was once a little girl named Fran who loved to watch her mother cook Sunday dinner. Every Sunday, Fran's mother would cook a ham. She would painstakingly make her glaze from scratch, cut the ham in half, and bake it slowly until it was crisp and delicious.
When Fran grew up, she prepared her mother's ham every week for her own family. One day Fran and her daughter were cooking together and her daughter asked, "Why do you cut the ham in half?"
"I'm not sure," Fran replied, "but it must be important. My mother always did it."
The next time Fran's mother visited, the three generations were cooking together. Fran's daughter asked, "Grandma, why do we have to cut the ham in half?"
Grandma blinked, visibly surprised, and said, "What are you talking about?"
"Come on, Mom," Fran laughed. "You always cut the ham in half. It's part of your recipe."
Grandma laughed. "I cut the ham in half because none of my pans were the right shape. It didn't fit unless I cut it."
The moral of the story: some traditions are borne of necessity, so continuing them doesn't always make sense. Tissue paper in invitations is one of those. Putting one in a non-engraved invitation can help it look more formal, I suppose, but the reason for the practice is mostly gone.