Answer:
The history of the metric system began in the Age of Enlightenment with notions of length and weight taken from natural ones, and decimal multiples and fractions of them. The system became the standard of France and Europe in half a century. Other dimensions with unity ratios[Note 1] were added, and it went on to be adopted by the world.
The first practical realisation of the metric system came in 1799, during the French Revolution, when the existing system of measures, which had become impractical for trade, was replaced by a decimal system based on the kilogram and the metre. The basic units were taken from the natural world: the unit of length, the metre, was based on the dimensions of the Earth, and the unit of mass, the kilogram, was based on the mass of water having a volume of one litre or a cubic decimetre. Reference copies for both units were manufactured in platinum and remained the standards of measure for the next 90 years. After a period of reversion to the mesures usuelles due to unpopularity of the metric system, the metrication of France as well as much of Europe was complete by mid-century.
In the middle of the 19th century, James Clerk Maxwell put forward the concept of a coherent system where a small number of units of measure were defined as base units, and all other units of measure, called derived units, were defined in terms of the base units. Maxwell proposed three base units: length, mass and time. Advances in electromagnetism in the 19th century necessitated new units to be defined, and multiple incompatible systems of such units came into usage; none could be reconciled with the existing system of mechanical units. This impasse was resolved by Giovanni Giorgi, who in 1901 proved that a coherent system that incorporated electromagnetic units had to have an electromagnetic unit as a fourth base unit.
The seminal 1875 Treaty of the Metre resulted in the fashioning and distribution of metre and kilogram artefacts, the standards of the future coherent system that became the SI, and the creation of an international body Conférence générale des poids et mesures or CGPM to oversee systems of weights and measures based on them.
In 1960, the CGPM launched the International System of Units (in French the Système international d'unités or SI) which had six "base units": the metre, kilogram, second, ampere, degree Kelvin (subsequently renamed the "kelvin") and candela; as well as 16 further units derived from the base units. A seventh base unit, the mole, and six additional derived units were added in succeeding years through the close of the twentieth century. During this period, the metre was redefined in terms of the speed of light, and the second was redefined in terms of the microwave frequency of a cesium atomic clock. Since the end of the 20th century, an effort has been undertaken to redefine the ampere, kilogram, mole and kelvin in terms of invariant constants of physics.
Explanation:
The metric system was and still is a very important part of how things are created, and therefor built.