Better grades – simple spelling and grammar mistakes could lose you up to 10% of your overall grade.
A professional edge – average essay looks immediately more professional when edited and proofread.
Authority – regardless of the arguments you make and included evidence, grammar and spelling errors undermine the paper’s authority. You don’t want your professor (or some other reader) to think: “Why should I believe him/her if they didn’t bother to edit their work/pay attention to mistakes”
Responsibility – if you submit an essay tainted by spelling and grammar mistakes, weak arguments, inadequate style and formatting, a reader may assume you’re irresponsible. Essay writing is more than justifying your arguments, you have to pay attention to every detail in order to get a high-quality paper