A design system isn't a file full of styles - it's a shared language between design and engineering. Once every component is defined once and reused everywhere, the team stops reinventing the same button on every screen.
In practice that means fewer hours spent aligning on small details and more time on the actual product. Branding, UX/UI and visual systems that look premium actually save budget, because decisions get made once instead of every week.
The payoff is biggest at scale: once a product has 20+ screens, consistency is the difference between an interface people understand and one that scares new users away.