Why do u think rates are usually written as unit rates

Simply because rates become easier to visualize when simplified.

It simplifies it down to how many this happens in that if you know what i mean. like miles per hour. if a car drove 300 miles in 5 hours thats the rate. 300 to 5 ... to simplify, divide 300 by 5. so youd know you were going 60 mph, and thats why unit rates are imprortant. do not copy this answer, change the words around a little bit