Why does a multicellular organism need specialized cells and why their apperances are different?

I'm not sure whether you are concerned about the appearances of the cells or organisms.

Unless the organism consists merely of a collection of one-celled organisms, the cells will need to perform different functions. The cells will be modified to serve these functions, which means that they will be different in appearance. A neuron looks much different than a red blood cell.

Of course, as the cells differ in appearance, the organisms will also differ.

I hope this helps. Thanks for asking.