When it comes to singers and to Tom Waits especially, the greatest impact is the one of their voices and not the one of their lyrics. It is the first impression an artist makes on a potential fan, and this first impression always counts.
As Tom Waits says, he has several musical instruments around his house, but the first one he speaks about is an early analog synthesizer, a Chamberlain bought from some surfers. And he speaks about this piece of technology because it is a primitive thing which can achieve wondrous feats.
Speaking of this, the question that pops into my mind is: what is more primitive, as a musical instrument, than the human voice? After all, it all started from here, through various forms of imitation – imitating sounds found in nature.
And Tom Waits agrees with this, since in his artist statement for Orphans: Brawlers, Bawlers & Bastards (from 2006) he says that the human voice is his favorite instrument. Only by using his voice, Tom Waits says that he can give personality to his songs, to make them belong to him even if afterwards they will be interpreted by other artists.
Furthermore, the artist says that writing a song without having a melody in your head gives you freedom. When writing poetry (because, in my opinion, this is a song without musical notes) you have the freedom to go anywhere you want. You are not afraid of missing a beat and it also sounds natural: after all, when writing the words, you are hearing only your voice and you are under the spell of only your feelings.
This way, the poems become. They are brought to life with the human voice and then they are enriched by the musical notes.
And when you are listening to Tom Waits you are certainly transported in his reality, in his life story. His voice makes you feel as if you can actually touch it with your hands and you can feel the denim. His voice can make you smell the smoke of tobacco and gasoline, his voice can make you feel as if you were the one on the road, always with a gas station close.
So yes, I say that the human voice is the best instrument a singer can use. All that comes afterwards, all that comes after the poem was written is just a means of selling it to the masses.