When you are building a website with a css framework like Bootstrap, you will only use a part of the framework and unused css styles will be included. Purgecss removes unused selectors from your css, resulting in smaller files. "With PurgeCSS, you'll have a hard time generating CSS that isn't under 10kb minified and gzipped." - Tailwindcss docs

Unused CSS is an online tool to remove unused CSS rules. It will check your pages, find unused CSS and let you download a clean CSS file.