Reusable, themeable building blocks. Define a piece of UI once, then use, override and nest it anywhere.
Create once, use everywhere
The navbar, footer and this button are all Builder Components. Edit the component once and every instance across the site updates. Below: three instances of the same Primary Button.
An instance only stores what it changes — styles, content, attributes — and inherits the rest from the component. Same component, three different overrides.