What role does SAT I and II play in admission? Their importance in selection?

Hi I'm a self-studying/homeschooled international student. I want to study for undergraduate program at a US university. I have completed GCSEs and A-Levels. I want to know how much importance do universities put on SAT I and II scores. What if my academic grades are not so good (Bs in A level), can good SAT scores make a difference? Thanks