The design system is a living whole that collects all the visual and interaction rules of a product in one source. It includes colors, typography, spaces, buttons, forms, and rules on how to use them. In short, it is the dictionary of your brand's digital language.
You may not need a design system for a small landing page, but if you have an ever-growing product that multiple teams work with, the system soon becomes chaos: the same button looks five different in five different places.
A well-established system earns three things. Consistency: the user sees the same language on each screen. Speed: new features are designed multiple times faster with ready-made components. Scalability: the team grows but the quality does not fall.
Design tokens are the core of this system; you define color not as 'blue' but literally (e.g., 'primary action color') and a change is reflected consistent to the whole product. The design system is an investment in the first place; but it is one of the fastest returning investments when installed at the right time.