The PSAT is a great primer for the SAT, and even the ACT, but it's more than just a trial run. PSAT scores are used to identify National Merit Scholars and award merit scholarships. More than 3.4 million high school students (mostly juniors and sophomores) take this nationwide, multiple-choice test every year.
The ACT and SAT are still used by private colleges and out of state colleges as part of their application process, so if you are interested in applying for a college that requires it, you will need to sign up and take the exam(s)