CSS Box allows you to assign a class and id to a stack. This can allow you to define your own custom CSS rules to apply to CSS Box and its content. There is also the ability to define inline styles as well.
Look at the following 2 CSS Boxes below and see how I can affect them using CSS. Since both of them share the same class, I can target both of them with the same CSS rule. However, by using the ID, I can target a single instance.
Look at the CSS tab in the page inspector to view the CSS. I am using very basic CSS here but it illustrates the use case for CSS Box perfectly.