The SAT, ACT, and AP are the most important exams for University admission
Levels the Playing Field
The SAT and ACT exams give Universities a way to evaluate students from dissimilar High Schools.
Universities do not know the difference between GPA's from various High Schools. Some High Schools are easier to get a 4.0 GPA - especially home schooled children.
Ranks Applicants
The SAT and ACT exams allow all students to be ranked. They are uniquely graded as a curved percentile relative to all of the other students taking the test.
The SAT median score will always be 1000. Half the students will always score less than a 1000, regardless of the difficulty of the exam. You are essentially competing against your peers.
Your peers/competitors are working now on prep!
Scholarships
Almost every college offers "merit based scholarships" to attract academic talent.
Their evaluation of "talent" is largely based on the SAT / ACT score.