Explain why prose must express complete ideas\

Prose must explain complete ideas because it enables the reader to comprehend the authors vision, and thought process as well as the emotional state that the author wants the character(s) to have. This is in the case of fiction.
When it comes to nonfiction it enables the reader to understand the material or the argument presented in front of him or her. Furthermore, arguments when presented with evidence which incorporates complete ideas and knowledge of the topic in question. It gives the reader a proper explanation and answers to the questions that may arise whilst reading the informative material at hand.