Part of the motivation was economic: armed horsemen, the dominant military units of the time, were expensive to maintain. The breakdown in the economy in early

Part of the motivation was economic: armed horsemen, the dominant military units of the time, were expensive to maintain. The breakdown in the economy in early medieval Europe meant that centralized rulers didn't have money to pay them. They gave them land instead, so they could support themselves in return for military service. You might say it was a medieval take on the military-industrial complex.