You have probably already seen global CSS files with thousands of lines that may influence anything. Change is too dangerous so you just add new rules. To the point everything must be rewritten.
That way you can prepare components that contains embedded styles that applies only for them. No complex CSS selectors are required – just use plain names:
At the same time you can import application wide constants using SASS so you do not lose ability to set colors, sizes, … in single place.
Usage in code is simple:
(btw. to avoid join operation you can use classnames lib)