Why is the II in copper II chloride important?

Answer 1

the II means that the charge of the copper, in this case, is +2, rather than, say, +1, which is copper's other possible charge.

the formula for copper II chloride is CuCl2, rather than copper I chloride, which is simply CuCl.

for all transition metals which have multiple charges, it is necessary to state the particular charge thet the atom is using.

Answer 2

The "II" in copper II chloride indicates the oxidation state of the copper ion in the compound. This information is crucial for understanding the chemical properties and behavior of the compound. In the case of copper II chloride, it tells us that each copper ion has a charge of +2.