Why would I use Next.js with React when I can just serve my React site as static html hosted on say Amazon S3?

There are several to choose Next.js vs React over serving a static HTML site hosted on a service like Amazon S3:

Server-side rendering: Next.js can render your React components on your servers instead of on your users’ browsers, improving your site’s performance and SEO.
Automatic code splitting: In a regular React application, the entire app is bundled into a single JavaScript file and served users/browsers. This means all components, routes, and functionality are loaded at once, regardless of whether the user needs them or not. On the other hand, Next.js can automatically split your code into smaller chunks and lazy-load them on demand, which improves the performance and speed of the site.
Routing: Next.js provides a built-in routing system that allows you to easily define routes for your application. Unlike React’s react-router, the Next.js is built for server-side rendering and you don’t need to use a separate library like the react-router.