As a fertilized egg divides , the cells differentiate because they

As a fertilized egg divides, the cells differentiate because they have to become [different] specialized cells to perform different functions.

Explanation:

The molecules of DNA of a human have to be duplicated over end over so that every human’s cell contains a copy of DNA that can be found in a fertilized egg.

When cell differentiates, the embryonic cell becomes a specialized cell. This leads to an expression of gens and that is what dictates the functioning of one cell.