Charismatic Leader is one of the first Diplomatic Policy Cards in the game, which conveniently becomes available right when you unlock access to the first group of more advanced governments that have the relevant slots. It is suitable as a "permanent" Diplomatic Policy all the way until Machiavellianism becomes available, because it will considerably speed up your Envoy production at all times and circumstances, which in turn will permit you much greater influence with city-states. In fact, you may keep using it well into the late game, when it will finally become obsolete. After all, charisma works equally well with the uncouth upstarts from early nations and experienced diplomats from mature civilizations!
German sociologist Max Weber defined charismatic authority as “resting on devotion to the exceptional sanctity, heroism, or exemplary character of an individual person, and of the normative patterns or order revealed or ordained by him.” That pretty well sums it up, for charismatic leaders shape the lives of their people, for good or bad, or sometimes both simultaneously. “Charisma” comes in two sorts: 1) a compelling charm that inspires obedience, 2) a divinely conferred power or talent for leadership. In ancient days the latter tended to be the case (at least, those barely civilized folk thought so); recently, it’s been all about charming the media.