The scale of things. Large projects take longer to compile and bundle because they're made out of thousands of files and hundreds of thousands of lines of code.

Yeah, your hello program will go just as fast without a bundler, or a simple bundler. It's when you go big, and use tons of libraries and stuff that it slows down and the advantages become clearer.

That's especially true when using big libraries like React and dozens of plugins for it, frameworks like Next.js, SASS for CSS or CSS in JS, all the JSX, all the TypeScript. It adds up especially if Babel is also involved to transpile for older browser support.

5 minutes is a bit of an extreme use case, but the point is after the first build, live reload would still refresh your code at the same speed. So working on one page you don't need to constantly rebuild the other hundreds of them, just the one you changed. If you target mobile platforms with Cordova or React Native then you also add a whole Android/iOS build + install + restart of the app. The same live reload would work even in that context, making the speed advantage even more apparent.

These things are designed for large enterprise projects, not for your small helloworld sized hobbyist programs.

Depends entirely on the bundler. They all have a watch mode, not all of them do hot reload. Hot reload is cool but full of gotchas, it's a hack but worst case you reload the page anyway. Some probably cache on disk, I think webpack can.

But if you think about it, you either want a clean build for production, or rebuild quickly during development. Watch mode does exactly what you'd do anyway, re-run a build. And IDEs can watch its output to give you error messages.

It's much easier to implement: just emit the code again when the file changes, and let it go through the pipeline. It's already there in memory all figured out, no need to figure a serialization format to put on disk to load back up.

But to circle back to the original question of why use watch mode when you can just rebuild it when saving a file: you're reinventing the wheel, and watch mode scales better and will always be faster. Yes, some people need watch mode.