[Klathmon]

Most of them!

The minimap extension is made easy because they can just throw some HTML in a side pane, the image preview plugin just leverages the image displaying abilities of the browser, custom styling is as easy as throwing some CSS in a file, the extensions which let you preview the web page you are working on right in the browser use the full extent of the browser, the plugin that lets you easily preview markdown just uses a simple markdown->html converter then just displays the HTML/css. Themes are just a set of CSS which targets certain classes. Plugins can move, change, hide, show, do anything.

It also let's plugin authors do things the original developers never imagined possible. 3D object preview using webgl in a custom pane, replacing the whole implementation of tabs with something else, completely redesigning how the sidebar works and looks because it's all just HTML.

And while none of that can ONLY be done in a browser, the fact that a browser is the UI means that it's much easier, much quicker, and much more maintainable.