It usually helps a person to get a better idea of how much or how big something is. I'm actually not sure why, but let's use an example. You got 38 out of 43 questions on a test. when you see 38/43 you don't really know how good that score is, but if you make it a percentage (Dividing the first number by the second to get the percentage) it's 88%. You know you did well on that test.