Why do we have different units for the same physical quantity?

Why do elements have different atomicities?

Why do we get rains?

As far as mechanics is concerned all systems of units are having the same dimensions for a given physical quantity. But for thermodynamics, electricity, magnetism and electromagnetics this is not the case. Why?

State about the correctness or otherwise of the following two statements. i) A physical quanity can have dimensions but no units. ii) A physical quantity can have units but no dimensions.

Why do we conserve biodiversity?

Why do elements in the same group have similar physical and chemical properties?

Why do elements in the same group have similar physical and chemical properties.