In some freshwater organisms there is a structure called a contractile vacuole that regulates water movement into and out of the cell. What explains the existence of the CV?

Freshwater organisms, including single cells such as Paramese or Amoeba, have a type of body called Vacuole, which by pumping out the amount of water enters the cell to extraneous cells from bursting because the cells with this organ lack walls They are cellular because they would have expanded if they had a wall