Consistency is one of the biggest advantages in Design Systems Engineering.
Recently I had a firsthand account while working on SnowflakeStories.com
The site is using the JAM Stack. The pseudo-back-end is created by a program called Ruby Middleman. The visual layer is using ZURB Foundation, a design system in its own right.
One thing you have to worry about as the size of your web application grows and ages is that of consistency. I have talked before about the dangers of going off brand. Maintaining a consistent system is one of the best ways to ensure you are staying on brand.
When we built the site we tried to extend simple components. Since we merely extended much of the existing ZURB Foundation framework, we could have a lot of consistency between our elements.
Rather than cobble together a melange of Stack Overflow, GitHub Gists and some engineer creativity I have built a robust system. When we need a new button, we have a component to use for that.
If we change how our brand colors are used or our components, this change is instantly rolled out site and app wide. The same goes for more complex user interface elements such as tabs and accordions. These are handled in the same way. If there is a change to spacing or colors, we already have it handled.
We get all of this for free. And with zero dev time.
This is because all we need is a few small variable changes and the entire site is regenerated based on the system we designed.