If you are willing to put in a little time to learn how GNU Emacs works, it can be an extremely powerful and versatile editing tool, as well as being used for almost any kind of text searching, replacing, or processing. Using Org-mode alone is worth learning Emacs. Once you learn the basics, they can be applied and used in all areas that you would use Emacs for. If you want a text editor and information tool that you can use for almost everything, then the learning curve is worth it. If you want simple text editor for editing small files, then Emacs may be overkill.
Readily available: Emacs is available on almost all operating systems. You can use it for free, and even use it for remote editing. It has a nice desktop application that anyone can use, even the ordinary user.
Lightweight application: Although it hasn't always been, Emacs is now a very lightweight, efficient editor that isn't going to demand much CPU power. This can be crucial for large-scale applications.
Easy learning curve: for complex editors, Emacs is one of the easier to get a handle on. There's good documentation, and an easy layout that makes figuring out how it works much easier than others.
Since Emacs is free software, support comes from online forums, and online searches. I have always been able to find an answer to any question I've had, as well as "How do I do XXX in Emacs?". I've found Emacs users in general are very willing to help others who are learning Emacs and to share what they have already done with Emacs.
Emacs can be easier to use than Vim as it is more similar to modern text editors. Its graphical interface is also better than anything Vim has to offer. Emacs also has better syntax highlighting and recognition of coding languages. Interestingly, Emacs handles clipboard copy and pasting into other programs a lot better than Vim does. Overall, if you know how to use Emacs, it is often preferred over Vim.
Overall, the ROI of Emacs has stemmed around its nature as a free, open source product. Usability is high, so when you use it you are more productive, but if you are unfamiliar with it, you will be less productive to start.
Emacs is more than just an IDE. It includes IDE-like functionality, but it is really much more than an IDE.
Emacs makes it easy for me to work in my environment, in a fashion to which I have become accustomed over the last 30+ years. It is more suited to the professional who has used it before.
