SAT scores are important because the exam tells colleges a lot about how ready you are to be successful in college. A standardized test provides a very easy comparison across students who came from high schools across cities and states. An "A" student in a less rigorous school might not be as ready to be a good college student as a "B" student in an excellent high school. SAT allow admissions officers to accurately compare across students.
This said, more and more schools try to put more weight on other factors - AP classes and other academic related activities (clubs, competitions, volunteering) that can provide a full academic picture of a student.
After this academic profile is compiled, things like essays and personal circumstances are used to provide a holistic picture. For example, colleges tend to be more forgiving about academic performance for athletes, because they have shown discipline and hard work at a very high level and give the college certainty that they can persevere through difficulties when they get challenged with all the hard academic work in college.
Bottomline, study as hard for the SAT and get as high of a SAT score as you can. It will help you for the rest of your life to get in the best college you can.