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.