The reasoning behind this is the heirloom movement. The old heirloom varieties have such wonderful flavor and excellent fruit(flavor-wise, size-wise, texture-wise) and they produce an abundant harvest PROVIDED disease doesn't get them. They have very little if any resistance to any disease or virus. So, commercial growers are now grafting the heirloom tops onto the roots of the more modern varieties. We grafted Brandywine tops onto Celebrity roots, which is resistant to almost all diseases of tomatoes. It is easy to do.
You have to have a little grafting clip like these, which I have ordered from Johnny's Selected Seeds. They come in a couple of sizes, and there are also clips for stems of squash and other thicker stemmed plants. You cut the top off the heirloom tomato, pinch the clip and slide it over the stem. Then cut the root stock, pinch the clip again and slide it over the stem, pushing the two stems together until they meet firmly.
Keep your plant watered, and IN THE SHADE. It takes about two weeks for the graft to grow.
Click here to check out the clips more thoroughly.