Why do we use percentages?

Percentages are often used. Because they can be easily compared to proportions and numbers different from a standard number. And for humans, the easiest ratio to compare is 100. Because it's a common denominator we often deal with, it corresponds to our most commonly used number system, the base of 10 decimal places. So instead of trying to compare 3/4, 4/5, and 1/2, write them down as percentages and get them at 75%; It is very easy to compare 80% and 50% respectively.