well because in order to get a common denominator you might have to multiply a fraction to get is, doesn't change the value of the fraction it's just written with greater numbers and is not in simplest form
meaning 1/2 is the same as 8/16 one is just not simplified.
ex.
3/4+5/6
(you can't add these yet because they don't have a common denominator)
but you can make a common denominator by finding the LCM of the two denominators
fx. 4 and 6 both go into twelve, so you'd multiply the entire 3/4 fraction with 3 (because 4*3 is 12) and the entire 5/6 fraction with 2 (because 6*2 is 12)
3/4 *3 is going to give us 9/12
5/6 *2 is going to give us 10/12
now that we have the same denominator we can add the fractions together
9+10= 19. so...
19/12