I can't tell you why. It's a basic skill that all live performers should know, though lazy singers probably want to lean on the skills of a sound-man. As a life-time stage performer with NO sound-man (most of the time) I learned to lean into the mic and back off the volume in order to get richness and bass response, and back off and really project to catch the upper registers. Leaning in and backing down the volume a bit gives you what's known as the proximity effect, which is very prominent in several Shure models and is carried to the peak (IMO) in some Sennheiser mics. I got my sister and her band hooked on the Sennheisers after using one to run a weekly open-mic jam. There were lots of Shure M-57s on sale after that!
BTW, you don't have to back away from the mic. Since most of the best vocal mics are somewhat unidirectional, you can continue singing and just turn away a bit. Alisson Krauss is a pro at that!