A mockup is a frame, not the artwork
The actual design — the logo, the label, the app screen — was finished before the mockup existed. A mockup is a separate step that places that finished artwork onto a realistic photo or 3D scene (a coffee cup, a phone in someone's hand, a folded box) so a client or audience can judge it the way it will actually be seen.
Why it matters more than it seems
A logo looking correct on a plain white background tells you almost nothing about how it reads on a real product, in real light, at a real size next to other objects. A good mockup exposes problems — low contrast, a detail that disappears at a smaller size — before they become an expensive printing mistake.
What separates a convincing mockup from an obvious one
Lighting direction and material texture matching the artwork to the scene, correct perspective when the artwork is warped onto a curved or angled surface, and shadows that make sense for the light source. A mockup that ignores any of these reads as fake even if the artwork itself is good.
What a mockup can't tell you
It shows how the design will look, not how it will be made — color proofing, material behavior, and manufacturing constraints still need to be checked separately before production.