As a musician (not a singer), I communicate with other musicians largely via body language, but the audience is largely oblivious to it. Only another musician -- and mostly just those who have played with me -- would understand the difference between lifting my bow 1 cm or 2 cm off the strings of my violin.
This is true, there certainly are many cases where the audience is oblivious. But imagine the stereotypical rock/metal band playing. Their stance, how the strut around, how they move, how their hair moves all effect how the audience reacts to the performance.
