When the SAT test was first used, it was a tool to help increase access to higher education. Over time, the SAT and ACT tests have had their share of controversy in regards to privilege and effectiveness. However, both are widely used by colleges, not only in admissions, but in graduation requirements and merit based scholarships.
Admissions: Students who desire attending a competitive school may need to earn a high score on one or both tests. (Many schools have median SAT scores above 1400)
Graduation Requirements: Some schools will waive or grant writing or math requirements when students earn high enough scores. For example, students who earn a 680 or higher on the Writing section of the SAT Reasoning Test fulfill the UC Entry Writing Requirement for UC Santa Barbara.
Scholarships: Many private schools offer automatic scholarships for SAT/ACT scores. Azusa Pacific, Biola and Grand Canyon University offer $3,000 to $17,000+ in yearly scholarships for SAT scores 920 and higher. (Minimum GPA scores may be required)
If you are looking to improve your SAT/ACT scores, the best way is to utilize free online resources and tests from Khan Academy, the College Board (SAT) and ACT.org. After that, I suggest Math & SAT/ACT review books you can purchase from local book stores or Amazon.com