Question

why do we have different units for same physical quantity?

Answer 1

Same physical unit could be too big or too small .
So, to avoid this problem we use different units.
let us take an example of distance ..
If the distance between two points is 1000000m , it will be too Lengthy and confusing to write the bigger unit . hence, we use km here .. 1000km.
therefore, there are different units of same physical quantity.

Answer 2

We have different units for same physical quantity because
1. Any quantity which can be measured are called physical quantity
2. It must be always comparable with the size of physical quantity to be measured.
3.The unit of any derived physical quantity can be measured as the product or quotient of two or more fundamental units.
Hope you got it