Voice is a form of expression we use mainly to speak, but also to groan and shout. These latter usages are the ones we learned long before speaking in our evolution, millions of years before. As they are more primal, they connect us with our body and expand the pleasure.
Our current way of life represses the use of the voice in this way, also during sex (which too is very old), and this causes effects usually associated with tension, such as anxiety, anorgasmia or involuntary ejaculation.
It is a much more important technique than it may appear, being essential for the treatment of certain diseases and to expand pleasure.