It's a classic. There is minimalism, simplicity, and elegance to it.
Emacs is available as a package to install on all flavors of Linux and can be installed with one command.
As they say with photography, "The best camera is the one that you have with you." If you're ever logged into a server remotely, (or for that matter, something really complex like: a firewalled server via SSH via another private subnet server via SSH via tunneling over SSH to a VPN server), you simply can't set up your IDE and have it be useful to you in Dimension X, when your IDE is only useful to you in your local environment.
Emacs is ultra-lightweight and was designed to run on the crappiest computers.
I can open, edit my file, save, and close it, and be off to testing/iterating on my product/feature/website/whatever I'm building before the other person has even found and navigated the file to open with his IDE.
With Emacs and a sufficiently large screen, I can split-window an arbitrary number of times just by using keyboard shortcuts. I regularly work with 2-4 files open in 4 quadrants; other times I have had 6-8 files windows (called buffers in Emacs) open.
Emacs is extensible with Lisp extensions, so you can get just about any new benefits of modern IDEs like code-complete, syntax and line highlighting, static checking, etc by installing plugins. I have my go-to Emacs customizations that I've been maintaining for over 15 years. (See GitHub repo: jontsai/dotemacs). There are also online communities sharing dotemacs files, e.g. http://dotemacs.de/
By being simple (unobtrusive, no fidgeting with mouse, extensive keyboard shortcuts, etc), Emacs and Vim allow you to get into the zone, that productive existence in the space-time continuum where developers can either a) code for 12 hours straight, or b) produce effective output for a day in 2 hours, cranking out code like a beast.
Terminal / Command line is awesomesauce. And Emacs can run from the terminal. Does your IDE run in the terminal? No? I didn't think so.
Macros in Emacs are really nice (you can record, replay macros).
Emacs is an IDE, but not in the sense that people think about IDEs. Review collected by and hosted on G2.com.