You can grow a tree from a cutting, but it's rarely done. Each variety grows in a different size and shape. By grafting the variety you want onto a know root stock, you get to choose both the variety of apple, as well as the size and shape it grows in.
As a young man, my brother and I used to t-bud (nobody serious grafts anymore) about 1,000 trees per day for most of June. We not rarely were part of a 10 crew team. It was a pretty good business until the EU expanded and the low wage countries took over the business.
