The most important issue to cover in an essay about the importance of voice is how voice works as part of proof by ethos.
Aristotle, in his Rhetoric, shows that we convince people by three types of argument, argument from logos (reasoning, logic), argument from pathos (manipulating the emotions of the audience) and argument from ethos (how we portray ourselves as speakers).
Your voice is an important aspect of how people assess your credibility. For example, if you write an essay with numerous spelling and grammatical errors, people assume that you are someone who is not very knowledgeable and has put little thought and effort into your writing, and thus will not take you points seriously. If your voice is authoritative and well informed, judicious rather than partisan, and uses sophisticated diction and syntax people will assume you are knowledgeable and be more likely to take your points seriously.