Intercostal breathing allows the performer to draw in more air without tightening his throat muscles. This allows him to sing or speak for longer periods of time, without drawing a second breath. Since drawing a breath is audible, singers prefer to avoid drawing breaths mid-phrase.
Since intercostal breathing leaves the throat muscles open, using this technique enriches and amplifies a performer's voice. This breathing technique also involves slow, even contractions of the diaphragm muscles, which slowly expels air and allows a performer to sustain the same tone quality for the duration of a phrase.
Since this breathing technique requires a performer to stand up straight, she looks better and more in control while on stage. Other breathing techniques tend to tire a performer out, which negatively affects their stage presence.