They don't. The reason to do it is not to save bytes, but because you have a dynamic page that needs to respond to the user without doing a full page refresh on every minor interaction.

Why on earth would you do a full page refresh on every interaction? Browsers have native support for partial page updates and prefetching.

JS is fine to use too as needed. Generally, it's not needed. Small snippets to pop open modals or flyouts of whatever covers a huge number of use cases.