Standardized tests are aimed to...well standardize. As I previously mentioned the difference in GPA between schools and classes can be pretty misleading. Hence, colleges use standardized tests to access the levels of teaching at your school and your ability as well since everyone will take the same test.
For example, a school has mostly A+ students in AP Calculus BC class but its students only score average on the SAT/ACT Math section, and vice versa. Something definitely doesn't add up here
Only a very very few colleges practice standardized-test-free admission (due to the fact that statistically, kids with higher households tend to do superior on these tests)so I would advise taking them at all costs, seriously. Standardized tests requirements change every year, so please check your schools' websites for most up-to-date.
Same thing with SAT Essay and ACT essay. They're optional for some schools but aren't for others.My recommendation is that just take them just in case. It's only an additional $15 and extra 30-45 minutes of testing time.
Personal story, I took the SAT 3 times and the one I got the highest score, I didn't take the essay because I was too fed up with it. Then I realized my top choice schools require the essay. It was definitely a lot of unnecessary stress. So just take them, please.
There are currently two standardized tests that colleges accept: SAT and ACT. SAT is overall more popular but here is the geographical distribution of between SAT and ACT test takers:
Before we start, I am much more familiar with the SAT and I am with ACT. In actuality, I only took the ACT once and decided it wasn't really for me so please take my advice on the ACT with a pinch of salt.