B&W is the faithful old method to conver a complicated message with a simple tonal changes. Sometimes a scenes have alot of colours which doesn't tell ur story. Your story may be simply a subject in the middle of the crowd. But the crowd is full of colours and drag away the focus instead. For example, your subject is a man standing in the middle of the crowd,but one corner is a women wearing is bright red. Well we all know red is a outstanding colour, but clearly she is not the subject, then colours is not helpful in bring out the subject. What you can do is make sepia, B+W or duotone etc....
drag the focus back using tonal value only to the center of the pic where your subject is.
There are times u also wanna just tell a simple action. The action is loud and clear, then colours is not useful here. Actions can be easily told using a B+W pic.
Contrast, black and white is the best contrast u can find. increase the contrast of the pic, you will cut down on the gray tones, then your blacn and white are outstanding to bring out the features of your subject.
That's my interpretation of B+W