It's a combination of things.

Adding polyfills for older browser targets.

Tree shaking, that is removing code that is not used.

Bundling several files into one to save on requests.

Transpiling, that is, if you want to write code in a different language to javascript eg. Typescript then vite will compile that to javascript.

Linting ie. Reformatting your code to standardise it across developers.

Vulnerability checking, are your dependencies out of date?

Execution of build tasks eg. Preparing static assets like images.

Packaging your app for deployment eg. Zipping

Including logging and hot reload triggers during development.

Bundling environment specific changes eg. Different backend urls if deploying to test envs.

Running unit tests on rebuild to avoid regressions.

Having said all that, the bundling for me is a disadvantage as my own code changes more than 3rd party libs and they are cached. I'm less fussed with types and a lot of the tests and env stuff can be done with a single-file express server for development.

The main advantage for me is that a buildless app will still work correctly in 6 months- permacomputing considerations.