The jazz lingo are sometimes perfect words to their solo, and the knowledge of jazz makes the music they sing more exciting. The vocalist feels the swing of the beat and begins the words of the song. Sometimes the vocalist will only scat a song. The vocalist may not have words, but is specially trained in scat singing. Scatting is also one way for musicians to share how a rhythm is played by using syllables that will create the swing beat.
Jazz music comes from the soul.
The beat comes from within each cat as they play their own specific part to the chart. When Cab Calloway began scatting it was said that he had forgotten the words to the music and began to improvise. To an outsider this may sound like gibberish, but to the musician it is the key to the soul of the music. Any syllable used has to represent the beat. Just like improvising on an instrument the vocalist's solo comes from the heart.