Prefabrication moves fabrication off the messy, weather-dependent, one-off construction site and into a controlled factory, where the same wall panel, stair unit, or bathroom pod can be produced repeatedly to a tolerance a site crew could never reliably hit. The gain isn't just speed. It's that quality becomes a property of the system, rather than a property of whichever crew happened to be on-site that day.
The joint is the hard part
Anyone who has designed prefab knows the panels are the easy part; the connections between them are where the discipline actually lives. A prefabrication system succeeds or fails on how well it standardises the joint — the interface, in the most literal architectural sense — because a strong module attached with an inconsistent connection produces exactly the fragility the whole system was meant to eliminate.
Component libraries inherited the same discipline
A design system's component library is prefabrication for interfaces: a button, a card, a form field, produced once to a specified standard and reused everywhere, so quality stops depending on which designer happened to build a given screen. And the hard part transfers directly — the component itself is rarely where a design system breaks down. It's the spacing and states at the seams between components, the digital equivalent of a badly detailed joint, that quietly reintroduce the inconsistency the system was built to prevent.
A library of good parts doesn't guarantee a good building. It just makes a good building possible without reinventing every wall.
Modularity was never really about the module. It was always about the joint.