Earlier quoted context omitted.
The same reason thousands of programmers use Vim or Emacs with various extensions as their development environment. It allows the same editing experience across multiple languages. A consistent interface, lack of need for learning many (often very different) IDEs, less proprietary annoyance, many many more options for customization, much easier to port your setup to various platforms, etc. Admittedly I don't think At…
I think that's less relevant than it used to be. JetBrains is now the de facto best IDE for Java, JavaScript, Python and PHP. It has Android Studio and even support for C#. Visual Studio also supports a wide array of languages. I'm in IntelliJ pretty much all the time, no matter the project.
Isn't it basically an Atom alternative? What makes it, and not Atom, an IDE rather than 'augmented text editor', to borrow GP's phrasing?