Answer: Cellular differentiation is a process of development of undifferentiated cells (having no specific function) into specialised cell (such as muscle cells, nerve cells) which serve specific function in the body.
A fully formed human infant is a multicellular organism having different cell types (with varied structure and function) that ultimately form various body tissues and then organs (such as kidney, heart, liver, pancreas, stomach, intestine) in the body.
These organs serve specific roles in the life of organism.
Thus, to accomplish different life processes and functions in the body, cellular differentiation is important for the development of a fully formed human infant.