| well I think that singing along with what I'm playing does several things for the music. Number one, as stringed instrument players we can play lines forever without ever taking a breath, so actually singing along with what your playing especially when you're soloing, will make you pause for breath and therefor naturally break up your phrasing into more melodic and musical statements.
It also helps me stay away from playing too many things over and over again, or from playing things that don't make sense musically. I'm generally hearing the ideas before I"m playing them, and to me the voice gives a certain pure quality to the sound that doesn't come naturally from an electric instrument.
I'm a terrible singer so I really don't tend to amplify it ever, it's more for me on the stage to keep a nice balance of phrasing and tone while I'm playing.
Easy,
Janek | 
