Turning is required for any directional piece of railroad equipment, such as most steam locomotives, or indeed many passenger trains, especially those that have a dedicated tail end car such as an observation car.
Individual locomotives and railroad cars can be turned on a turntable, but obviously whole trains cannot. A wye, or a loop, are the only ways of doing that.
In British railway terminology, a wye is known as a triangle, which is much more logical.
Railroads in America have way more wyes than railroads elsewhere, and American locomotives and cars are much more likely to be directional than those elsewhere. This is due to the fact that in most places in the United States, the railroad came first, or at least early, and therefore had much more freedom to lay down tracks where they wanted. In Europe, extensive use was made historically of bi-directional tank locomotives and push-pull trains, and more recently most diesel locomotives and electric locomotives ordered in Europe have been fully bi-directional.