We have different units for the same physical quantity because these units were used in different parts of the world. Every country had its own way to express these physical quantities which started creating confusions. To avoid these confusions, the standard unit system was created which is known as SI units(International System of Units).
For example, consider the units of mass:
1 tonne = 1000 kg
1 slug = 14.6 kg
1 pound = 0.45 kg
The SI unit of mass is kilograms.