Here's why:

* Structural Support: Cellulose forms the cell walls of plant cells, providing them with structural support and rigidity. This allows plants to grow tall and withstand external forces.

* Abundance: Cellulose makes up a significant portion of a plant's dry weight, often exceeding 50%.

* Importance: Cellulose is a vital component of plant tissues, giving them their characteristic strength and shape. It's also a key component of wood, paper, and other plant-based materials.

While other molecules are important in plants, such as carbohydrates, proteins, and lipids, cellulose is the most abundant and plays the most significant role in plant structure and function.