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
Fractions can't be added or subtracted if they don't have the same denominator. It doesn't have to be the least one, but they both have to be the same one. ... I can't think of a reason "why" that's short enough to type on an Android keyboard, but I'll give you this to think about: If I asked you to add 1/2. and. 1/5, and you didn't want to bother finding a common denominator, then what WOULD you use for a denominator for the sum ? ?
