42/57 as a percentage is 73.68%. This is the rounded value you get when you turn the fraction into a percent with two decimal places.
The fraction 42/57 can be simplified by dividing both numbers by 3, giving 14/19. Even though the numbers are smaller, the percentage stays the same because the value of the fraction hasn't changed.
Knowing the percent helps you compare this ratio to other numbers quickly. For example, if a test score is 73.68% you can see it's just under the typical 75% benchmark.
