Why are there so many types of cells in one organism? what kind of work do they do?

There are multicellular organisms. They have a wide variety of cells. Cells that have the same origin and function are combined into tissues. Tissues form organs. The cells that form different tissues have a certain structure, which is determined by the functions of the tissue. But you also need to remember that not all multicellular organisms have a variety of cells. Lower plants do not have any organs and tissues, so they consist of the same cells that perform the same functions. Such plants are algae.