Because you'll have 29 different margins, 67 different left paddings, etc all over the place. Tailwind allows you to create a consistent design system. CSS was supposed to be "write one class, use it everywhere instead of inlining". Usually it ends up with nobody reusing that class and everyone creating their on in the same codebase. There's an article I don't have a link handy, which explained how ridiculously many classes were used by GitHub/Lab, Airbnb, etc. Now even GitHub have their own utility based CSS system and others are following along, including Bootstrap with their CSS utility classes.