Because cells are tiny, and a microscope is needed to view them, and even when you use a microscope you don't necessarily get a good view as cells can be damaged, and many parts cannot be seen without an electron microscope, which you are unlikely to find in most normal schools. With a replica, it can be made an easier size to see, and all of the organelles can be made and placed inside, to give the best view of what the cell looks like.