It hit me when I had to google a translator for tailwind. Why do I use tailwind if I already know CSS? Then later it hit me again, why is it called cascade styl

It hit me when I had to google a translator for tailwind. Why do I use tailwind if I already know CSS? Then later it hit me again, why is it called cascade style sheet - it's supposed to be separated the style from markup. And well why use an entire JS engine to rewrite some CSS.