Holidays are all about tradition. The wider culture builds some core traditions, but each family or group that celebrates together also builds their own unique traditions. "We always have orange danish rolls on Thanksgiving morning." Why, Grandpa? "Because your great-great-great grandpa liked them. It's tradition." There's an old story that illustrates this phenomenon.
Then there are the lesser-publicized but goofier things that go into Thanksgiving that arose in order to accommodate a great number of family members and to liven up the celebration. Penn Holderness wrote a little song about these common Thanksgiving memories, and has photographic evidence. You'll probably recognize some of them from your own experience. Rickety folding table for the kids? Hide an overcooked turkey with gravy? Lawn chairs at the table? Take pictures in funny hats? Mac and cheese because the kids won't eat anything else? These are the things we do to get everyone together. We may pretend the holiday is about gratitude, or the food, but it's the company that makes Thanksgiving.