Here's why:

* The cell membrane is selectively permeable: This means it allows some substances to pass through while blocking others. Water is one of the substances that can pass through the cell membrane.

* Osmosis is the movement of water: It's the passive movement of water molecules from a region of high water concentration to a region of low water concentration across a semi-permeable membrane.

* The concentration gradient drives osmosis: Water moves to try and equalize the concentration of solutes (dissolved substances) on both sides of the membrane.

So, osmosis is a vital process that happens at the cell membrane, enabling cells to maintain their internal environment and function properly.