CSS-in-TS libraries such as Stitches (opens in a new tab) and Vanilla Extract (opens in a new tab) are fantastic options for building type-safe UI components; taking away all the worries of class names and StyleSheet composition...
You may need full control over your StyleSheet output. Your job might require you to use a framework such as Tailwind CSS. You might just prefer writing your own CSS...
Creating variants with the "traditional" CSS approach can become an arduous task; manually matching classes to props and manually adding types.