Take nature as an example, where the goal is to portray reality in all its elements; colour an obvious one.
Some subjects are also inexorably linked to colour photography. Take nature as an example, where the goal is to portray reality in all its elements; colour an obvious one. But shooting in black & white offers a whole other world of views in the wild. For evidence that it can work exceptionally well, check out the remarkable output of Nick Brandt.