Butter or solid fat tenderizes and aerates the cakes.  It tenderize by coating some of the gluen-forming proteins in the flour.  Unsalted butter produces the best flavour, not onlly because of its own incomparableflour by also because is release the flavoursof other ingredients more fully.  A cake with less butter will seems less sweet.  Margarine or other fats do not release flavour as well. 
Butter will hold the max amount of air if its temperature of 65F (18.3C) to 75F (23.8C) when beaten.  During baking, the melting fat makes the batter more mobil because fat is insoluble.  It dispersesinto tiny particles throughout the batter.