Theory:

Need for standard unit
In the olden days, scientists used to conduct experiments and record their findings in their own systems. But, it was inconsistent and lacked uniformity since it varies from the other scientists. Moreover, they were unable to organise the results of other people's experiments due to a lack of communication. As a result, they devised a system for taking measurements that would be consistent.
 
System of units
Scientists recognised the importance of standard units for physical quantities at the \(11^{th}\) General Conference on Weights and Measures in Paris, France, in \(1960\). As a result, the International System of Units or SI units was declared as a common standard unit all over the world to express the quantities of measurement.
 
Important!
The word 'SI' is abbreviated from the French word 'Systeme International'.
Base quantities and units
There are seven physical quantities known as 'Base quantities' or 'Fundamental quantities' for which a standard unit was developed to measure them. Those standard units were termed as 'Base units' or 'Fundamental units', which are given below.
 
8th measurement(physics).png
 
Important!
In the system of units, the CGS, MKS and SI units are the metric systems of units. But, the FPS system is a British system of units.
Importance of SI unit
The Mars Climate Orbiter was launched by the National Aeronautics and Space Administration (NASA) in \(December\ 1998\) to collect data on the Martian climate. Unfortunately, on \(September\ 23, 1999\), the Orbiter vanished while approaching Mars at a very low altitude, marking the end of a nine-month mission.
 
Mars_Orbiter_Mission_-_India_-_ArtistsConcept.jpg
 
According to the report, the orbital calculations were inaccurate due to a communication error between the mission navigation team in California and the spacecraft team in Colorado. One team used the MKS system of units for calculations, whereas the other team used the English FPS system. This misunderstanding resulted in a loss of approximately \(125\ million\ dollars\).
Reference:
https://upload.wikimedia.org/wikipedia/commons/c/cd/Mars_Orbiter_Mission_-_India_-_ArtistsConcept.jpg