In an absolute sense, it isn't. It's just another form of measurement.
But it is now the most common measuring system used throughout the world.
The above answer is wrong. It is EXTREMELY IMPORTANT to science as it is a standardised and regulated set of measurements without which it is difficult to deal with very large and very small quantities.
A tragic example is the loss of the Mars Observer which went out of control because one group of engineers (the guys who designed the propulsion system) used one system of units and another group (the guys who calculated the flight parameters and the fuel to burn to get the spacecraft into orbit around Mars) used another set of units.
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