Here's why:

* Structural: Cells are the basic building blocks of all living things. They are the smallest unit that can carry out all the processes necessary for life.

* Functional: Cells are responsible for all the functions that keep an organism alive, including:

* Metabolism: Taking in nutrients, converting them into energy, and eliminating waste.

* Growth and development: Dividing and producing new cells, and coordinating their activities.

* Reproduction: Creating new organisms through cell division.

* Response to stimuli: Reacting to changes in the environment.

* Homeostasis: Maintaining a stable internal environment.

While some organisms are single-celled (like bacteria), multicellular organisms are composed of trillions of cells that work together in a complex, coordinated fashion.