Why do we ask rhetorical questions? We ask them to frame an idea. The point of a rhetorical question is to make you ask me that question so that I can answer it.
Consider the first sentence in this post. When you read it, my goal is for you to say, "I don't know...why do we ask rhetorical questions?" Then you keep reading and find the answer. I have planted the question in your head to garner your interest and focus your thought.
People often say that "rhetorical questions aren't supposed to be answered." In fact, that's not quite true. A rhetorical question is supposed to be answered by the person who asked it. If I ask a question and I don't have an answer to it, it's not really a rhetorical question; it's just a regular question. It may be a pointed question or a relevant question, but if you aren't using it to frame your ideas, then it just doesn't count.
It's easy to tell people not to put questions in their essays. And maybe for the very beginning of writers (like elementary school) that's ok. But it is far more valuable to allow people to ask questions in their essays, so long as they actually understand how they are supposed to function.