Manager of the Economy
One of the major reasons for calling delegates to Philadelphia in 1787 was to resolve economic problems arising from the Articles of Confederation. We expect our presidents to maintain prosperity, resolve disruptive strikes, keep employment full and the various markets healthy.
Even though his power to control the economy is actually quite limited, woe to the chief executive who governs during an economic downturn and is perceived as ineffectual or indifferent. The politician in each president knows what it takes to remain popular. In the words of political consultant James Carville, "It's the economy, stupid."