Our bodies are designed to breathe in through the nose. It filters it, adjusts the temperature, releases nitric oxide, etc. Mouth breathing causes many health issues, including tooth decay. It is difficult for me to image singing “naturally” if I must breathe unnaturally. Also, in my research I have found that most of the elocutionist authors from the 1800s were fans of breathing from the nostrils. I could probably dig up some sources if you’re interested.