Why do we need to have common denominators when adding fractions?

So that we have the right answer once we do the addition

Say that I measure my self at 12 feet with one measuring stick, and go on with another measuring stick that tells me I'm 3 meters tall, if I added those, I would be at something like 15/46 or something like that. See that would be wrong. We would want the right denominator so that it could be easier and also correct.