In the soap testing experiment, what is the purpose of adding some MgCl2 to the soap solution at the end?

The water is hard due to the presence of ions of Mg²⁺. Now in hard water soaps are ineffective. Since in hard water Mg²⁺ ion forms precipitate, which concludes less number of soap molecules are present in the solution and less amount of froth which disables the cleansing property of the soap.

Now the experiment, the very last step is the hard water test where some amount of MgCl₂ is added,

Now this magnesium salt act as a source of Mg²⁺ ion and now the soap action can be determined, whether it is able to form froth or not.