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

Define the $SI$ unit of force,the newton $(N)$. Define the $CGS$ unit of force,the dyne.

The new $SI$ unit of mass $1 \, kg$ is defined in terms of the difference in the masses of two ${}^{133}Cs_{55}$ atoms. One of these atoms is in its ground state and the other is in an excited state that has a frequency of excitation close to $9.2 \times 10^9 \, Hz$. The number of atoms required to get $1 \, kg$ of mass this way is of the order of (Planck's constant = $6.63 \times 10^{-34} \, J \cdot s$; mass of proton = $1.67 \times 10^{-27} \, kg$; Avogadro number = $6.02 \times 10^{23} \, \text{particles}$; speed of light = $3 \times 10^8 \, m/s$).

$1 \text{ kilowatt hour} = \dots \text{ joules}$

Match the items in Column-$I$ with their corresponding values in Column-$II$.

Column-$I$	Column-$II$
$(1)$ Light year	$(a)$ $3.08 \times 10^{16} \ m$
$(2)$ Parsec	$(b)$ $9.46 \times 10^{15} \ m$
$(3)$ $1 \ AU$	$(c)$ $1.496 \times 10^{11} \ m$

Mention the unit of power in the British unit system.