The metric system was developed by a variety of European thinkers over quite a long time, starting with the development of the idea in the late 1500s, to its adoption in most of the world in the 1900s. This system, along with SI, has been a tremedous boon in that a scientist located anywhere in the world can immediately understand the results published by another scientist in their field, because they are all using the same units.
However, the first usage of the metric system was not in science, but actually trade. We have all heard about the story of defining a foot as a king's foot, and how different kings could lead to different definitions of a foot, which is a nightmare! This is why the meter and are (you might be familiar with hectares) were among the first units defined; the size of the land you bought wouldn't change based on who was king. Also, another issue for that time was the different measures of weight in terms of payment. Remember that until the early 1900s most of the world used gold and silver as money, and if your business partner overseas used a different weighing system than you did, and you were unfamiliar with that system, he could easily rip you off. Or, you might have to pay a hefty fee to have a professional converter get the measurement right. But with a metric system, your gold in France was worth the same amount in England, Spain, or Russia, and you could be confident that your measurement at home would be the same abroad.
Scientists also took notice of the metric system early in its growth, because for the same reasons that traders wanted to trade in the same units, scientists want to be able to read results in the same units. As an American scientist, I can tell you that it is a real pain to have to convert temperatures such as Farenheit, Celcius, Kelvin, and Rankine! Or between horsepower-hours, Joules, and ergs for energy. Having just one system of units makes our lives a lot easier, even if we like our old units!
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