Let us define “commerce” as the buying and selling of goods on the wholesale level. Commerce includes such things as trade between large companies and trade between nations. We can argue that this is the backbone of industry and other business activities because it creates a way in which those activities can be made profitable.
Without commerce, there would be no reason for industry. If there were no commerce, a firm that had a factory, for example, would have to try to sell its output in a piecemeal fashion to small consumers in its own immediate vicinity. This would be very difficult and so it would not be possible for industries to remain highly profitable.
Commerce, then, is needed in order for industry and large businesses to be able to exist and to be profitable. Therefore, it is the backbone of these other activities.