Have you ever had a friend taking the same class as you but with a different teacher? Even though the class has the same name, the difference in teachers can make them seem totally different. Have you ever complained that you got the "hard" teacher while your friend got the "easy" teacher? Some teachers give out lots of A's, and some give out only a few. This varies a lot with different schools as well. Some schools are "easier" and give out more A's than others. So comparing students by GPA alone isn't quite fair. That's why colleges like looking at the SAT. Everyone takes the same exact test, so it's easy to compare people no matter what kind of school they went to or what kind of teachers they had.

The main reason colleges look at an SAT or ACT score is to get a universal standard from which to judge their applicants.

Schools also look at GPA and your classload right? Well, depending on where you go to school your GPA could mean something totally different from someone at another school. Some schools weight honors or AP classes, which completely change GPAs. Some schools are harder, some easier. All this leads to the fact that GPA isn't standard across the pool of applicants a college receives.

The SAT and ACT are universal. They are the same no matter where in the world you take them, and they're always scored the same way. It may not be the ideal system, but when you've got an SAT score of 2100 with a GPA of 3.0 versus someone with an SAT score of 1800 but a GPA of 4.0, it will help you out.

I hope that helps!

another note, extracurriculars are hard to compare between schools.

i think the big advantage the SAT offers is universal standard