If nearsightedness is from a too-long eyeball, and farsightedness from too short, what are bifocals benefits?

Lense changes: The lens of the eye hardens year by year from birth to about age 52 when it becomes a rock. The impact of this in a practical sense starts at age 38-42 when near vision becomes a problem. Bifocals compensate for this hardening of the lens and allow for both near and far vision with one eyeglass. This is independent of the basic power and occurs in all humans/all refractive states of eyes.