What attributes of rome's citizen soldiers made them so important to the republic growth?

Roman soldiers were citizens of Rome. Legionaries were well trained and extremely physically fit and robust soldiers. They were well supplied and supported. Legionaries were promised that after their service to Rome they would be granted land as a form of pension. This giving of land to former soldiers grew the empire as new lands were needed to give to the retiring soldiers. It had the effect of placing well trained ex soldiers in newly acquired territory.