The ACT writing test is becoming more and more popular as time goes on, but only some colleges require it. It might seem confusing, but in reality, it's not.
Depending on your major and the college you wish to attend, the writing test may or may not be required. To find out if your desired college requires it, go to the ACT website and look up the schools you're applying to.
To get the most up-to-date info, you should also contact that school's admissions department to be sure of their ACT requirements when you submit your application.