Why do you need a common denominator when adding fractions?

because you can not add unlike things
To get the right answer. It's not immediately obvious what the sum of 1/2 and 1/3 is. But if you realize that 1/2 = 3/6 and 1/3 = 2/6, then 1/2 + 1/3 = 3/6 + 2/6 = 5/6