Here's why:

* The cell membrane is essential for the cell's survival. It acts as a barrier that controls what enters and exits the cell, maintaining the cell's internal environment and allowing for essential processes like nutrient uptake, waste removal, and communication with other cells.

* It defines the boundaries of the cell. The cell membrane separates the cell's internal components from the external environment.

* It provides structural support. The membrane helps to maintain the cell's shape and integrity.

While there might be variations in the structure and composition of cell membranes, the fundamental function of separating the cell from its surroundings and regulating transport is universal to all cells.