The effect is based on the fact that both the original salt and the other added chemical have one ion in common with each other.
Due to the increase in concentration of H+ ions from the added HCl, the equilibrium of the dissociation of H2S shifts to the left and keeps the value of Ka constant.
The very pure and finely divided precipitate of calcium carbonate that is generated is a valuable by-product used in the manufacture of toothpaste.
The soaps precipitate due to a combination of common-ion effect and increased ionic strength.
Sea, brackish and other waters that contain appreciable amount of sodium ions (Na+) interfere with the normal behavior of soap because of common-ion effect.
Many transition-metal compounds violate this rule due to the formation of complex ions, a scenario not part of the equilibria that are involved in simple precipitation of salts from ionic solution.
It occurs because as the total ion concentration increases, inter-ion attraction within the solution can become an important factor.