I am not going to write a post on the specifics of Emacs, or the wonders of org-mode, there are plenty of resources available on the details of how they work, but I will touch on the reasons of why I keep using them, and returning to them.
In its bare form, an org-mode file is a plain text file that adhers to an outline structure. There are headings and subheadings (as deep as you want to go). The headings kan have keywords attached to them, which can turn them into action items, agenda items etc.
Org-mode supplies ways to search the files, present the search results in different views, and allows you to act upon the items.
Over the years org-mode has been extended to a veritable swiss army knife of functionality. Within a text file I can have tables and calculations, I can embed code (and execute it), export parts of the documents as nicely formatted PDF files (or any other format for that matter).
Tables with calculations can serve as input to other applications, which in turn can present the results of their processing within emacs. For instance I keep track of my weight in a table in org-mode, calculate trends and averages, and send this data to gnuplot which creates a nice SVG plot which Emacs and org-mode then can present inline.
I can write articles and notes, easily restructure the documents, and export them as a beautifully typeset document using LaTeX and pandoc.
The great thing is that all of these tools are free. Emacs has been around since the early nineties, and org-mode since 2003.
While other applications look more spiffy, and have beautiful bells and whistles, none of them come close to what I can achieve with Emacs and a text file.
I can also have the same setup on my Mac at work and my PC at home.
The one area where the functionality is lacking, is on the mobile side. There are apps to view and edit org-mode files on phones and tablets, but not offering the native experience. Since I spend most of my days in front of the computer, I am willing to take the less-than-stellar mobile experience in return for the functionality I get on the computer.