Originally, the Imperial system of Measurement was based on that used in the Roman Empire, and during that time, all were standardized. After the collapse of the Empire, the definitions of the measurements began to wander a bit until by the 18th C, they were completely different in different countrys and often different in separate regions of the same country.
Such was the situation in France at the time of the Revolution, and it was the need for a new standard that brought about the introduction of the Metric System. Eventually other countries adopted this new standard as well. It should be noted that in the century since its adoption, the length of the meter has been redefined a number of times until a standard that was based on a real figure could be rationalized (I believe it's currently something like "the distance that light travels in 1.2 x 10^-9 seconds").