Some "writing rules" can be recognized as spelling, punctuation, capitalization and grammar. These rules help students clarify and understand their essays. It is worth learning these writing rules when the audience can finish reading without stopping to figure out what they really intended. Readers won't read your essays if they find gramamr mistakes because it would make them "boring".
Writing conventions such as spelling, punctuation, capitalization, and grammar help make a student's essay clear and understandable. When the audience can finish reading, without having to stop to try to figure out what was actually intended, the value of learning these writing conventions becomes clear.