Without a system, every site grows into fifteen variants of the same button.
One defined set of building blocks: colours, text sizes, spacing and components, with agreements on when to use what. Design and code refer to the same source, so they don't drift apart.
Not for a single standalone page. As soon as several people work on it, or the number of pages grows, it quickly becomes the cheapest option: every new page is then a matter of assembling rather than designing from scratch.
A system nobody maintains is ignored within a year because it is no longer accurate. Agree who decides on additions, or you will end up with fifteen buttons anyway.
A question or project on this topic?