I've heard Emacs referred to as a Lisp run-time that happens to implement a text editor within it. You can reprogram it within itself without stopping to parse, recompile, or reload any code. This is pretty normal for Lisp-based applications.

One interesting library I've come across is skewer-mode. It depends on a package that implements an asynchronous web server to inject some handy javascript into your browser that opens up a two-way channel with Emacs. I just have to add the href to load the script in my file and then I can open up a javascript REPL in emacs and load code into my browser without refreshing or anything.

I've heard of projects that let you programmatically embed and control webkit instances.

I publish books from emacs.

It's a handy programming environment and not merely a single-purpose application.