Why take and prepare for the SAT Subject Tests?
Schools that require SATs feel that they're an important indicator of your ability to succeed in college. Specifically, they use your scores to help make admissions and placement decisions. Like the SAT, the SAT Subject Tests provide schools with a standard measure of academic performance, which they use to compare you to applicants from different high schools and different educational backgrounds. This information helps them to decide whether you have the academic ability to handle their curriculum.
Your scores may also be sued to decide what course of study is appropriate for you once you've been admitted. A low score on the Math Level I test, for example, may require you to take a remedial Math course. Conversely, a high score on the Chemistry test may exempt you from an introductory science course.