Why fresh water protozoa bear contractile vacuole?

It lacks a wall thus use the contractile vacuole to collect and expel excess water from the cell.

Explanation:

The contractile vacuole regulates the quantity of water inside a cell through a process known as osmoregulation. Osmosis constantly drives water in the cell since it is enclosed by a semipermeable membrane that can not keep the water out thus it is needed to prevent rupturing of the cell due to excess water in the cell.