Why is understanding academic writing rules important for students?

Many students struggle with academic writing even when they understand the subject well. One common reason is not being fully aware of the dos and don'ts of academic writing, such as maintaining a formal tone, structuring arguments clearly, and avoiding unsupported opinions. I want to understand how following these basic principles actually impacts grades and overall readability. For example, which rules matter the most in assignments and research papers, and which mistakes are often overlooked by students but noticed by instructors?