The clock during the exam counts down. Let's say you're on question 32 with 26 minutes to go. How much time can you spend on the remaining questions? Less than a minute? More than a minute? Wasting time trying to figure it out aaaaaaaaaaaaaaand PANIC! Hyperventilate. Poop self.
But if you're going backwards from 48 on down, you could be on question 32 with 26 minutes to go and instantly know that you have less than a minute per question and can then correctly pace yourself.