Cubic zirconia exists because the optical properties of diamond turned out to be reproducible in a laboratory. It's a synthesized crystalline form of zirconium oxide, engineered to simulate diamond's appearance.
Built For Ease
The clutch back's position on the post is a small adjustment with a visible effect. Close to the lobe: flat, snug, forward-facing. More post exposed: gap forms, angle shifts, the earring starts to hang rather than sit. The back's placement is doing design work most wearers never think about.
Visually Speaking
Post findings are used wherever the earring's front element is intended to sit flush against or close to the earlobe, such as in studs, clusters, button earrings, and drop earrings with a rigid top. They all rely on the post's perpendicular projection to keep the piece in its intended position.