Great question. For me, Emacs ended up taking the place of a number of tools I use daily:
1. Text editing (obviously). spacemacs provides nearly any nifty trick that vim would give you for text editing (and like I said, I'm a believer in the way of modal editing).
2. HTTP client. Emacs has a package called "restclient" that allows you to write what amount to cURL commands - in plain text, with documentation - and execute them in-line within my editor. Other teams I've been on use Postman, which is great, but I like the fact that I can save the file and commit it alongside our services/tests/etc.
3. Note taking & task lists. Org-mode is Emacs killer app. Honestly, I'm still pretty fresh to this whole "being organized" thing. But already I'm finding myself writing more and more notes, TODOs, etc. and it's really helping me on a new project that is complete chaos right now.
4. Clojure IDE. Cursive for IntelliJ comes close, but nothing beats the experience of doing Clojure development in Emacs. Since my team adopted it as our daily workhorse language, I've never had a moment where I wanted to leave Emacs. It's integration with the REPL and LISP in general makes it the most powerful programming environment I've used to date (YMMV).
There might be more... the point is that Emacs is more like an application platform than a text editor, and it has some really powerful applications that have been developed for years. I now have this strange urge to do more and more inside of Emacs, if only because it gives me a common interface into my workflow.
EDIT: Forgot, magit for doing git operations. Great interface and I only (very very rarely) need to drop down to terminal to do things now.