i think everyone will give a different reason to your question.
heart and rational? well to me, with b&w, it is all about capturing the mood. works great with portraits, people on the streets, certain events shots and some landscapes even. there are some things where colors actually are distracting. b&w comes in handy when capturing a certain mood, feel, importance, aspect... u know that something special about that frame at that time.
well having said that, i do know of ppl who hate b&w and stick with colors only. hence, as said earlier, very personal.
Tones.... in black and white, everything is now portrayed in tonal differences of white, grey and back....
It's easier to pick up a difference of such tones than in colour sometimes. And also when the BG is smothered in lots of diffferent colours which may drown out your subject, B&W may be the option to save the picture.