The train set is the fundamental element of model railroading. Rokal sets contain everything necessary to get started in model railroading, although most sets found in the North American market did not include power supplies.
Of course, once you have the train set, and have run the train for a while, many people get the yearning for more track, more locomotives, more cars, structures, scenery items,........
Needless to say, many train sets are “subsidized”, which is to say that the individual components in the set cost less than if they were purchased separately. The manufacturers do this with the hope of downstream sales of additional items.