This is one of the first Wildcards in the game, available quite early. However, unless you're playing as Greece, it may only be used after you develop Political Philosophy and get the respective slot in your government. It is a nice way to get one of the early Great Generals, even without building an Encampment. This can be really useful for a militaristic civilization bent on early conquest, or it may be a pure waste of effort - it all depends on your current strategy, and the neighbors you got. Sometimes you won't even need a General to conquer them; other times you simply won't make it without one. So, you should think carefully whether you really need a Great General so early - if you don't, then there's no reason to use Strategos.
In the Hellenic world, a strategos meant, literally, “army leader” … although over time it came to mean a “great general.” When the Macedonians overran Greece, it was used by Philip II and his son Alexander for commanders on detached assignments as quasi-representatives of the king. Later, during the Diadochi empires, it was a gubernatorial office combining civil and military duties. In Greek-speaking Byzantium, a strategos was the supreme military officer of a field army, usually groomed from birth for the semi-hereditary position.