Q:
A: Why? Because a body of people like Congress cannot cannot run everything, too many voices trying to make *every single decision* necessary to run the government would bog down the government and no decisions would ever get made. Having a President allows a branch of government to act when Congress cannot and allows legislation passed by Congress to be carried out, without the worry of a couple hundred people bickering over how it should get done. An easy way to look at it, is seeing the President as someone who speaks for the many voices and carries out their decesions.