Here's why:

* Barrier: The cell membrane acts as a barrier, separating the cell's internal environment from the external environment. It controls what enters and exits the cell, keeping harmful substances out and allowing essential nutrients and waste products to pass through.

* Selectively Permeable: The cell membrane is selectively permeable, meaning it only allows certain substances to pass through. This helps maintain the cell's internal environment and protect it from damage.

* Protection from External Threats: The cell membrane helps protect the cell from harmful substances like toxins, pathogens, and physical damage.

Of course, other structures like the cell wall (in plant cells) and the capsule (in bacteria) also provide protection. However, the cell membrane is a fundamental protective structure found in all cells.