Cellular differentiation describes how different cells perform different processes and have unique functions. There are at least 250 cells in the human body and each one plays a different role, and all of them are crucial to the body's ability to maintain homeostasis. For example, liver cells produce proteins that are important in blood clotting. If these cells are damaged, blood will not clot properly. If blood does not clot, a body cannot maintain homeostasis and the person will fall ill.
TL;DR: every cell plays its part and keeps the organism alive.