Because many web sites and apps aren't as simple as "my first homepage" and don't only consist of first-party code. Think component libraries. Reusable code. Content management systems. Third-party SDKs (chat widgets, support widgets, payment widgets like Stripe, etc.).
One of my earliest webdev jobs was at a company whose product was a widget you could add to your site by adding our `<script>` tag. Thus, our CSS needed to coexist with the first-party site's, not to mention any other third-party widgets on there. In other words: the same exact reason you need modules in traditional languages.
One of my earliest webdev jobs was at a company whose product was a widget you could add to your site by adding our `<script>` tag. Thus, our CSS needed to coexist with the first-party site's, not to mention any other third-party widgets on there. In other words: the same exact reason you need modules in traditional languages.