Question

Why do all freshwater organisms need contractile vacuoles?

Answer 1

The freshwater organisms are hypertonic and their surrounding water is hypotonic.
As a result water keeps rushing into the organism throughout the day.
So, they need contractile vacuole which pumps out the water by osmoregulation, thereby balancing the osmotic pressure.

Answer 2

Because of the cellular environment of a freshwater organism being hypertonic, the water diffuses inside the cell constantly and gets collected in thecontractile vacuole, which squeezes the extra water out of the cell periodically.