Very important. If you are preparing for SAT, or any other exam, then maybe reading comprehension is one of the 4 parts, but it aids to other parts too. If you read more, you would eventually develop a good vocabulary and you can easily improve your writing and grammar skills too.
If you're not the greatest reader, you need to improve, to enjoy reading will mean you can learn anything you want, if its a struggle, then learning will become a struggle too, and nobody needs or wants that... The better you read, the better understanding of all things, and the better your understanding, the easier all thing will be!