The Inverse Square Law
Useful when setting up a microphone, the inverse square law states that, in a free field the intensity of sound drops by 6 dB for each doubling of distance from the source. On the other hand, the opposite is true when you move closer to the source.
If you start with your lips on the mic, you will experience a greater dynamic range as well as the proximity effect and more plosives. Movements away or side to side will decrease your volume proportionate to the amount of distance you move.
Conversely, if you start at a comfortable distance from the mic, you might not achieve the volume needed to rise above your bandmates (we're looking at you, drummer!). You'll also be left floating in front of the mic, unsure whether or not your angle and distance are the same as during soundcheck.
So vocalists are left with the choice of "eating the mic" and suffering the consequences of the proximity effect and plosives, or, floating in front of it and hoping they stay in position.