Most often it's done because of a developmental problem where one leg segment has come out slightly shorter than its counterpart on the other leg, affecting gait and posture. Only one or two bones need to be lengthened if the patient is lucky. Shortening the other leg is probably also an option, but I figure people would want to do something to the affected leg, rather than muck about with the "healthy" one.

There is at least one instance that I recall where someone born with a form of dwarfism had all four limbs - all twelve bones - extended to "normal" length. As to whether it was strictly ethical to do that is an entirely different matter, considering the patient was a child.

I mean, it's definitely the best time of life to have the lengthening done what with bones being greener and still growing anyway, but the patient wasn't exactly in the position to be making an informed decision about whether they wanted to go through it.