The dominant political organization for most of the history of civilization, or at least the past several thousand years, has been the empire, with the defining characteristic of empire being the incorporation of diverse peoples and cultures in one political unit. Empires were built by conquest, by one group conquering several others.
The development of farming permitted land to support many more people than can be supported in nomadic or more basic economic systems, but it is a curious fact that for much of history, the relatively numerous farmers have been the conquered rather than the conquerors. For much of their history, the empires of Asia and the Middle East have been ruled by foreign elites, steppe warriors from central Asia and their descendants.
So why do we see this pattern conquest by groups that are much less numerous, usually technologically more primitive, and far weaker economically? One theory is that the harsh conditions of nomadic life made for more skillful and fiercer warriors. Later European conquerors brought the military skills of ocean going pirate-traders.
A slightly different point of view is that the key advantage the nomads (and later the Europeans) had was simply mobility. Nomads and traders can wander, but farmers need to stay with their fields.
This fits well with the idea that the Indo-Europeans who spread their language over much of Europe and Asia several thousand years ago based their expansion on the domestication of the horse.