Proofreading is absolutely necessary. Even seasoned writers and authors have others look over their work. Imagine reading a published book with grammatical and sentence structure errors; what would be your impression of the author? Most likely negative.
There are times when you may commit careless errors and the only way to catch these errors is by proofreading. If you hand in or publish a piece of writing without looking it over, you risk minor errors slipping through the cracks. Proofreading your own work will help you gain a better understanding of the kinds of errors you are prone to make. The more you proofread your work, the less you will make the same kinds of errors. After writing your first draft, give yourself at least 1–2 days away from your writing before looking it over. Taking a break from your writing allows you to look at it with a new perspective.