Admissions tests are often used as another way for universities to choose the best candidates. Not all courses require admissions tests - the most notable are courses like medicine and law. Though not strictly an admissions test, some universities also make use of the STEP papers to decide between candidates, though this paper is now limited to mathematics. (The AEA papers available in some subjects has resulted in STEP papers for subjects like chemistry and physics being phased out.)
Even if an admissions test or additional exam exists for your subject, you will not have to take a test unless your university says so. Check the appropriate prospectuses and websites.
Select an admissions test to learn more about it.