Why is Machiavelli so important?

Machiavelli is hugely important in the history of political philosophy; indeed, in the history of ideas in general. Machiavelli is arguably the founder of modern political science, in that he was the first to treat politics systematically as a wholly secular discipline. Prior to Machiavelli, politics was always treated as an outgrowth of Christian morality or theology. Machiavelli, however, as a Renaissance humanist, found his inspiration not in the lives of saints or in the Bible, but in the great historians and political writers of antiquity such as Livy.
Machiavelli's approach to politics was brutally realistic. He wanted to show politics as it really was, as it actually existed in the real world, not how it ought to be. To both contemporaries and subsequent generations alike, Machiavelli's secular approach to politics was nothing short of scandalous. In his masterwork The Prince, Machiavelli makes a number of recommendations as to how a territory's ruler should gain, and maintain, political power. His advice is often disarmingly frank, urging the necessity of acting with dishonesty and ruthlessness when the time is right.
The word "Machiavellian" has entered into the English language to describe a style of politics characterized by a complete lack of moral scruple. Machiavelli's position was actually more subtle than this characterization would suggest; but he did believe quite strongly that if a prince was serious about holding on to power, he needed to put all accepted moral and religious standards to one side, and do whatever was necessary, however ruthless.