Analysis of water in a place shows that the water contains Magnesium Chloride. The people in that place are advised to use detergents for washing clothes. Why?

hardness of water
Calcium and magnesium dissolved in water are the two most common minerals that make water "hard." The degree of hardness becomes greater as the calcium and magnesium content increases and is related to the concentration of multivalent cations dissolved in the water.
Synthetic dtergent are much better cleaner than the soaps as they are made up of many petrolium products and so they have antibactrial nature too and they can easily displace the calcium and magnesium ions of hard water and so it can be used in hard water too.