Question

why do we have different units for same physical quantity b?

Answer 1

many physical quantities can have same units. this is because the units are used for measurement purpose. large units are very much convenient.
eg:length can b measured in diff units though the quantity is same.... jzt because we get clearer idea of accuracy.

Answer 2

Magnitude of any given physical quantity may vary over a wide range, therefore, different units of same physical quantity are required.
For example:
1.Mass ranges from 10-30 kg (for an electron) to 1053 kg (for the known universe). We need different units to measure them like miligram, gram, kilogram etc.