Why do you think rates are usually written as unit rates?

It's so you can compare two rates

For example, 
person A walks 10 miles in 2 hours
person B walks 40 miles in 4 hours

If you want to know who is the faster walker, then you divide the distance over the time
person A's speed = distance/time = 10/2 = 5 miles per hour
person B's speed = distance/time = 40/4 = 10 miles per hour

So we can see that person B is faster as 10 is larger than 5. This is only possible by comparing the unit rates of 5 mph (for person A) and 10 mph (for person B)