The SAT essay doesn't affect your score, but it is used by some colleges as an independent measure of your writing ability. However, it is very easy to do well on this essay if you follow my template, which addresses everything the graders are looking for!
With just a few days of practice, you will be able to score at least 16 on the SAT essay. If you do exactly as my template suggests, you will score over 20 for sure.