Because having a common denominator means that you can perform functions on the fractions as if they were 'normal' (by that I mean integer) numbers. You can't subtract 1/8 from 1/4 because their denominators make them completely different numbers. However making the denominators of both 8, by multiplying the numerator anf denominator of the first fraction by 2, gives you 2/8 - 1/8, which can be done. As much as I'd love to give you an analogy for it, I can't think of one, so you just kind of have to take my/the teacher's word for it
