This has to do with how effective your paper is. Naturally, this depends on what you set out to do. Different kinds of writing may try to persuade, amuse, inform, entertain, alarm, or inflame the reader. Purpose is one of the most important aspects of writing because different goals are accomplished in different ways. This is why there can be guidelines for good writing, but no rules. When you're wondering whether it's "Ok" to do something in an essay, ask yourself whether or not it helps you to accomplish your purpose. Organization, Style and Mechanics, Focus, Coherence, Audience, and Development are all functions of Purpose: that is to say, they are important for a reason. When you are given a writing assignment in a class, you are basically being given a purpose or the parameters of a purpose, e.g. write an arguing essay. Therefore, it is very important that you understand the assignment.