Here's why:

* Cells are the basic building blocks of life. They perform all the essential functions necessary for an organism to survive.

* Different cells are specialized for different tasks. For example, muscle cells are designed to contract and move, nerve cells transmit signals, and red blood cells carry oxygen.

* This specialization is achieved through the expression of different genes. Each cell type has a unique set of genes that are turned on (expressed) and off, resulting in the production of different proteins and ultimately different functions.

So, while all cells share some basic similarities, the specific function of a cell is determined by its unique genetic makeup and the proteins it produces.