I do serious singing and one major thing is the volume of air you inhale. By using your diaphragm and holding your chest up you can use a much larger volume of air. No one normally breaths like this since you don't need it for normal life but if you need to sing out a not for a minute or more (and yes sometimes you actually have to sing a note for a minute or two without breathing) you want a heck of a lot of air. It isn't easy to breath really well and takes some practice but I've found in harmonica playing it is very rewarding to learn to do it right. And what you call "chest breathing" isn't needed to inhale quickly, you can breath in just as fast if not faster by using your diaphragm and abdominals.
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